It seems backward to force a reason for this guy to be using this symbol to kill people when the symbol itself doesn't seem to have any reason to exist. The whole draft reads like a very, very detailed description of a comic book villain or bad guy from a supernatural action movie; the character itself isn't bad, but this isn't really telling an interesting story, and I think it's because you're focusing too much on the how (how dangerous it is, how the symbol changes to prevent inoculation against it, how intense the security procedures need to be, etc) rather than the why (why does this symbol exist, why is it on this guy).
"Why does the guy use it to kill things" is a step in the right direction - but if this guy killing a bunch of things with this symbol is the entire story, it's not going to be interesting, whatever his reasons are. The fact that you're reaching for flimsy plot devices like "it's a compulsion effect" or "he's just a psychopath" is a clue that the overall situation needs to be thought out more so that reasons for him to do things can develop naturally.
If you want to make this work as an SCP, I think you'll have to come up with a convincing plot to explain it better. Maybe it was able to trick him into believing something that would justify all the murder? Why does the symbol want to kill people in the first place instead of, say, propagating itself or covering everything in polka dots?
A backstory about how this guy ended up with this symbol on him and discovered what it could do might be a start, though I'm honestly not sure how viable this concept is as the center of an SCP. As a bad guy in a tale or something with its own plot that he's just a part of, sure, but an article that's just about him and his ability is going to be a tough sell.
On another note, I'm having a hard time buying into this because it seems like if the anomaly is the symbol and not the guy, they could just sedate him and cut his arm off or put him in a medically induced coma as soon as it gave them a hard time.
