He was convicted for murders that were committed 26 years ago. In Chinatown, one of my favorite movies, the villain says "Politicians, public buildings, and *****s all get respectable if they last long enough." That's what happened here.
Add in some airheaded Hollywood activist types that fell under Tookie's spell, and before you know it he's a celebrity. His victims are long forgotten, and the horrendous damage he did as founder of the Crips gets minimized.
If anybody deserves to be executed, this guy does.