Riggs turned into such a pansy by the final Lethal Weapon its almost hard to remember he was a violent, suicidal wreck in the beginning. He was the kind of cop who would snort coke, drink on the job and encourage a hostage taker to shoot him. He was a broken man, which is understandable following the death of his wife. Riggs is a Jekyll And Hyde character in the original, capable of being friendly one moment before transforming into a massive lunatic the next. This clearly stems from his emotional turmoil, where he isnt even sure if he wants to exist anymore. Then hes partnered with a nice, boring family man in an attempt to balance him out, which doesnt do much to curb his recklessness - like throwing himself and a jumper off a roof and into an airbag - but with the healing power of friendship hes able to work through his issues and see that live is worth living. Righteously murdering the villains also helps.