While on the subject of that welder, he and the other fake FBI agent had the upper hand in that apartment scene earlier in the movie. From the welder's perspective, there was no reason to even give Jesse his third of the share. He was prepared to kill for it, and Jesse was defenseless at that point (gunshots would make noise, yes, but you didn't have to use a gun).
Obviously we sympathize with Jesse as the main character and wanted him to live, but just saying the welder kind of blew it twice in this movie by making bad decisions.
No, the first time he spared him was justified