What's wrong with my Minecraft world?

Ok, so last summer I built a huge city in Minecraft. It had villagers in it, and about 3 iron golems. I played on this world two days ago, and it was fine, but logging on today I got horrible lag. Looking for the reason, I turned around and saw around 200 Iron Golems in a large group, taking up half the city. I also had this problem with a large mass of Villagers. I have no idea what to do. I've tried dropping Splash Potions of Harming on them, but the lag is so intense I can't even do that. I also have no memory of spawning this many in.
I really don't want to delete this world, but I can't do anything about the problem. Can someone help?

It looks like someone or something hacked into your comuter or griefed it. IF this is not the anwser then your computer is really bad.

You will have to kill them all. Make sure all settings are on low as possible to make less lag. You can also install mods to kill them though.

if you get essentials it is /killall [mob type] <radius>

if you get world edit it is /butcher [radius]