I think that if you see her through the mirror, the effect is diluted to where only if you inadvertently caused a death, you would die. But I think because she physically came out of the mirror that anyone she looked at would automatically die, whether or not they had done something. She has the power to kill anyone; she just CHOOSES to go after people who have had a part in others' deaths. So that's why they were affected.

i only know why Sam's eyes bleed...is because the dream he had where Jessica died and he didn't told anyone about it....Dean idk...

cos they kill demons and the odd person all the time :)

if you hide a secret,of death, bleeding is a sign that you or is hiding something, sam bled because he hid a secret of death, which was a vision of dying jessica

Sam's eyes bleed because of his belief that he killed Jessica.

We have not yet found out why Dean's eyes bled.

Kripke's said that there is reason as to why Dean's eyes bled. He'd said we may or may not find out the reason though.