Tip is clearly going through a ton of emotions after Nipsey's murder, but still joined us Monday on 'TMZ Live' to offer his theory behind WHY anyone in the rapper's beloved Crenshaw neighborhood would kill him. Ironically, he believes it's Nipsey's amazing outreach to his community -- countless efforts to improve lives -- that sparked a jealous rage inside the shooter.

T.I. says the guy who pulled the trigger is probably trapped in a mindset of hating to see someone from his block, or someone who looks like him ... advancing in life. The sad part being ... that's exactly the kind of person Nipsey tried to uplift.

Check out T.I.'s breakdown of what he sees as a senseless crime ... he points to jealousy and envy as possible motivations -- but it's deeper than that, according to him.

Like The Game, T.I. says he's disgusted and disheartened by Nipsey's killing.

Despite life-saving measures by paramedics, he was pronounced dead at the hospital. Law enforcement sources tell us cops are looking for a black male in his 20s, who escaped in what appeared to be a waiting vehicle after he rolled up by foot.

In the meantime, T.I. -- like the rest of L.A. -- is awaiting answers.