To have so soon 'scaped world's and flesh's rage,And if no other misery, yet age! (7-8)
The "rage" associated with the world and the flesh suggests death and killing. The speaker seems to imply here that there is something death-like or murderous about life itself. From this perspective, death offers a release, a kind of life. It, at least, doesn't "kill" us the way the "rage" of the world does.
"For whose sake, henceforth, all his vows be suchAs what he loves may never like too much." (11-12)
The speaker vows not to like anything too much. In a way, he's sort of killing off some his emotion or passion, putting it to death. This is kind of weak, don't you think? He doesn't want to invest in anything too strongly, from an emotional point of view, because losing something is so terrible.