Ravenholm in Ruins
As you can see from the outset, things have not gone on well in Ravenholm - the headcrab shell and legs hanging from a rope tied to the tree prove it. Your destination is the house at the back on the right; the one with the entrances covered up with boards. There's a zombie out here; use the gravity gun to grab an explosive barrel, then put a bit of distance between you and it and fire the barrel to take care of the zombie. You'll need to use the gravity gun a lot in Ravenholm to conserve ammo; there are an enormous amount of zombies, but also lots of handy things you can fire at them, too.
Use the crowbar or gravity gun to get rid of the boards and head inside. In here, you'll notice a zombie's torso on top of a large sawblade, and a lot of propane tanks on the wall for you to ignite zombies with. The next room, blocked off by a bench and a few dug-in sawblades you can pull out with the gravity gun, is full of zombies, so have fun blowing them up or cutting them in half or just bludgeoning them to death with the bench. It's generally a good idea to carry a sawblade with you in case available throwing items suddenly run out and you don't have time to pull out a gun.
Some zombies will approach from the dark corridor - use a sawblade to dispatch them. In the next dimly-lit corridor, use the explosive barrel on the right to take care of the zombies approaching from around the corner, but retreat as this will ignite some more explosive barrels out of sight which will blow you up if you remain there. Once it's clear, head on through. Outside, to your right is a bit of a dangerous obstacle - a propeller blade on an engine, constantly spinning around. It's pretty easy to figure out that if you approach this standing up it's not going to end well, so grab your sawblade and crouch underneath it. Some more zombies will approach from the building ahead; you can either dispatch them now whilst crouching with your weapons, or retreat and let them walk mindlessly forwards and get chopped to bits by the propeller blade. Once they're dead, crouch past and head inside the building. In the room, push away the wooden pallet, and watch out for the headcrabs and zombies coming at you from the dark corner of the room. There's another fixed propeller blade in this room which you can spin yourself using the gravity gun's punt, or simply dispatch them another way. Head outside, and use the explosive barrels to blow away the zombie approaching from the alley. At the end of the alley is another building; go inside and take out the zombies and headcrabs. Follow the path outside towards the flaming pile of dirt with various bits of zombie impaled on spikes, and you will be rather impressively introduced to a new ally.
"Another life to save?"
He won't really do much except help deal with some of the approaching zombies with his shotgun, and promise he'll "keep an eye on you". However, you'll thank him later. At the moment heading right is fruitless, other than killing the zombies in the boarded up house (don't forget to use the sparks and gas tank to set the caged zombie on fire), or turning off the flames at the top of the road to access an area filled with zombies and an electrified ladder you can't currently get over. So, instead, head around the edge of the fire, dealing with the zombie behind the tower as you go. Turn the gas valve to close it to stop the flames, and enter the before concealed doorway behind the macabre display of zombie pieces. Inside this building, use the sawblade, explosive barrel and propane tank to deal with the zombies, then approach the two large machines up the stairs. You'll see a hastily-sprayed lambda symbol on one of the pillars - if you look at the machine next to it, above is a vent grate which you can remove. Climb up and inside the vent and follow it; you'll re-emerge outside and you'll find an extra SMG grenade and normal grenade on a higher-up ledge above the zombie display. Go back inside and this time find the junk-filled staircase. Use the gravity gun to get rid of the wardrobes and stuff, and use the explosive barrel to take out the headcrab and zombie at the top of the staircase. Turn left and work your way through the rooms killing the various enemies until you reach a window and lever overlooking the electrified fence and ladder. If you haven't already killed the zombies below you can do it from a safe place up here; just use a grenade. Pull the lever to turn off the electricity, then go back down and around to the fence, which you can now climb over (watch out for the poison headcrab ambush on the way out).
Climb the various ladders, and at the top, hop across to the roof of the other building. Up here, your violent friend will appear again on a high ledge, and inform you on being vigilant and that he has traps you can use. From here, look down and you'll see a series of wooden planks serving as a walkway across the road below. Jump up and crouch through the window at the end, and inside, throw a grenade down to clear out the zombies and headcrabs. Make your way outside. Out here, you'll see the first of the aforementioned traps; a car held up by a winch. Release the brake, and a few tons of metal slam into the ground. So go ahead - coax some of the zombies in the area beyond to it, and laugh as you pull the lever and they all get mashed by the car. Deal with all of the enemies then observe the second car trap. Use the brake to pull the car up, first of all, then use it again to get the trap to work properly. However, in order to progress you need to use the trap as a lift - release the brake to have the car drop, then jump on it and ride it up to the wooden walkways above the ground. At the top, use the walkway to approach the building, and your friend will finally introduce himself; he is Father Grigori. FATHER?! That's right, this is a religious man who is walking around setting up traps to mash and maim his former chapelgoers and shooting them with his super-powerful shotgun. Oh well, crazy world, eh? Jump in the building using the window, and open the door. Go down the corridor to get a loading screen.
Open the door at the other end and deal with the fast headcrab that comes at you from the raised corridor. The explosive barrels have no practical use here, so just ignore them. In the cramped room up ahead, Grigori will kill a zombie outside the window from the building across, and there's another dormant zombie in the corner which you can burn with the propane tanks. Use your pistol or SMG to take out the poison headcrabs on the far window ledge, and when you climb out be wary of the three zombies on your right. Use the cinder blocks on the far ledge to take them out, or wait for Grigori to. Below, there are an awful lot of headcrabs and zombies, so shoot some of the explosive barrels and the combo effect should help to take out most of them. Throw some other stuff down to take care of any survivors, then drop down at the top of the road, in the dead end to the left of the window you came out of. Shoot the explosive barrel behind the grate on the left, and you'll release some burning fast headcrabs - deal with them, then wait for the fire to die down and crawl inside for some pistol ammo and a few medpacks. Take care of any remaining enemies and climb the ladder at the bottom of the road, in front of the large barricade.
Walk along the metal girders up here and you can see Grigori kicking and shooting some more zombies out of the building you saw him in. To the left is a small lambda-symbol marked attic with some supplies and a fast headcrab in it, and to the right is another room with a legless zombie inside. Through here you'll find some benches with assorted zombies and headcrabs strewn about them, as well as a few medpacks and a sawblade. Grab it and approach the stairs down; there are a large number of zombies down here, so use the sawblade and follow it up with a grenade or SMG grenade to clear them out, then head down, around the corridor and through the door for a loading screen. Afterwards, the room across from you contains some more medpacks and some SMG ammo, then head right and out the door to emerge outside again. You'll see on the rooftops a new and very annoying adversary, the fast zombie. The ones on the rooftops will just jump across and disappear out of sight, but one will come up from down the street ahead of you. For now, use your SMG to deal with them; a better weapon is available soon.