the physical pain lifeforms experience when dying or being murdered doesn't bother me. i'm troubled by things that seek to destroy the soul.
i saw a documentary called grizzly man, about a bear lover who lived in the alaskan wilderness to study bears. he anthropomorphized them and turned them into his buddies. one day one of the bears attacked him and his girlfriend and ate them. the entire slaughter was caught on camera (which wasn't shown in the film, though the director was given access to the audio of their slaughter and you can see him in agony as he's listening to it).
but it didn't bother me that much. their bodies were being destroyed by the bear, but not necessarily their souls. as harrowing a death as it undoubtedly is to be eaten alive by a bear, it doesn't compare to those crimes, violent or nonviolent in nature, that seek to destroy the soul.
the kind of stuff that really bothers me is pedophiles who rape prepubescent children, pornographers, women being raped in front of their children (generally i find sexual crimes to be the most unsettling). sadistic actions that seek to kill the soul. that's what gets under my skin. but physical pain, in and of itself, doesn't present problems for me.