A while back I stumbled across the Lincoln memorial and ended up talking to Leroy Walker. IIRC, he gave me the task to sell him artifacts, and possibly asked me to locate Hannibal, although I'm not certain. I put the quest to the back of my mind (they had gone off on a hunt but it was tediously long so I gave up on seeing what they were doing) and now I come across the Temple of the Union to discover that the slavers kill them all and I fail the quest. So my question is, is this my fault or is it a bug? It's irritating because if I had known this was going to happen, I would have acted more.

It's all your fault.

And now you have to live with the blood of those poor people on your hands.

How can you live with yourself?

But I didn't even pick any option, I didn't know anything, what is this I don't even. <_<

There are several paths through the quest, one of which is selling out the slaves to the slavers.

However for that to happen I think your character needs to know where the slaves are. So maybe you told the slavers and forgot, or maybe it was a bug. More info:

http://fallout.wikia.com/wiki/Head_of_state

Ok, I think it must be a bug - he first asked me where (or what) Paradise Falls was, I said no, (probably irrelevant) and I think he tasked me with finding Hannibal/the escaped slaves (sketchy memory), but I had no idea of their location and so I don't think I possibly could have told them where they were. And I have no memory of doing so. So hopefully the 'setstage' command will put it all right, thanks.