Except with Sever, you need lifesteal on something else to make it work. Without lifesteal it's pretty useless. With life on kill, that's all you need. So you can find two weapons with life on kill (which is really cheap to find since no one wants it) and you got the same thing.
Does Sever have any random props? If it does.. having one with lifesteal built in would be the gold find of the century.
you can find one with lifesteal, but you'd end up doing more damage finding one with either Strength, Crit Hit Dmg, or a Socket