Can disturbing images or footage cause mental issues?

In the past i have messed around online with certain sites such as liveleak and bestgore, out of curiosity i have viewed certain explicit footage such as beheadings, executions, shootings ect. At first my reaction was shock and disgust and the more i kept on watching the more i became used to it, i got no enjoyment out of watching it but i was always curious about understanding the bad that goes on in the world, i wanted to know the calibre of both good and evil, so i continued to watch more and more of it until i had to stop, this was about 3 years ago when i was 19, now that i'm 22 there was something my brother was showing me on youtube, a deputy got into an exchange with a Vietnam veteran and there was no blood or anything really gory, it was the gun shots and the sounds of someone pleading and screaming for their life, this time it was different, my hands started shaking and my heart started beating out of my chest, i had to leave the room, i am confused because i've seen things that would be considered horrific and mind altering, but i didn't have this reaction, can watching things in the past change you into having a stress response to anything remotely disturbing?