Well the boys have killed a lot of things, maybe it doesn't matter if it's actually human? Oh and don't forget, when you kill a demon who's possesing someone, you also kill the vessel! They're no saints and accidents happen

Sam had a prophetic dream about Jessica but thought nothing of it. When it came true, he felt as if he had killed her, and kept it a secret, that's why Mary came after him.

Kripke said he would explain why Dean's eyes bled, but so far we have not heard of any humans Dean has killed and then kept it a secret, so...

I agree with everyones answers to why Sam's eyes bled it had to do with his viseon. But I'm not sure about why Dean's did,the best thing I can come up with is that he could have killed someone who was possesed.
And this probably isn't true but it could be that when Dean went back in time in In The Beginning he did know that his mom and dad would die and he didn't warn them, and that happened in the 70's I think, so technically he already did that.
In a weird way that could be what caused it, though I'm not too sure about what had to happen for people's eyes to bleed so I'm not really certain.

I only know why Sam's eyes bleed...it's because of the dream he had about Jessica's death, he knew but he never told anyone....
Idk about Dean's though

Sam's eyes only bled cause bloody mary said it was Sam fault for killing Mary Winchestor or his mother.

well the curse might have been stronger when she got out of the mirror so i didn´t care if they had caused someone elses death