Why did they

have bombs at their homes?

What do you think? Did they forget those? Or did they want to kill their parents too? I read in the 11k that they found 2 bombs at Dylans room.

No, I don't believe they wanted to hurt their parents. They probably already had enough stuff to carry and couldn't hold anymore. They already had 2 guns each, the bombs for the Cafeteria, the diversion bombs, a few bags full of Molotov's and other pipe bombs. Plus bombs in each of their cars and they also had other kinds of weapons such as knives.

They had a ton of stuff on them that day. It isn't unlikely that some things would get left behind. Also, wasn't 'Vodka's Vengeance' found at his house? I don't think he had any intention on actually using that the day of the massacre. I think he left that one behind on purpose.

I think these were not so much bombs as these Co2 "firecrackers", possibly damaged, incomplete or "test types" too.

Remember they built a lot of explosives in several batches and we have Eric's notes saying that not every batch is very good. My best guess is that it was really "leftovers" from their production, or examples of the poor batch that they decided not to use in the end.