Zoom wants the Flash's speed, so he needs to catch the Flash and rip his speed.
But, as was implied in many episodes he cannot steal someone's speed if they are dead.
So why is Zoom sending metahumans to kill the Flash?
Killing Barry isn't Zoom's goal1; it's worth noting that, once Barry actually makes it to Earth-22, Zoom is very unhappy at two of his underlings taking out their frustrations on the Flash:
[Deathstorm and Reverb are kicking the bejeezus out of Barry]
Killer Frost: Stop, both of you! Zoom wanted him alive. You know what Zoom will do if you don't obey him.
[Zoom appears and kills Deathstorm]
Zoom: That. [he rounds on Reverb] What did I say you should do if you encountered a speedster? Leave him unharmed. Does this look unharmed to you?
The Flash Season 2 Episode 13: "Welcome to Earth-2"
Zoom's actual goal is to develop Barry's abilities even more, by pitting him against all of these powerful adversaries.
Earth-2's Harrison Wells makes this observation in "Running to Stand Still", and Zoom confirms it:
Wells: I know why you keep sending villain after villain from our Earth to this one. You want Barry to get faster. You want him to get stronger. You want him to become more powerful, more full of Speed Force. You're fattening him up like a Christmas goose.
Zoom: The more power he has, the more I can take
The Flash Season 2 Episode 9: "Running to Stand Still"
Telling his underlings to kill the Flash ensures that they'll put maximum effort into their task, and being in mortal danger is the ultimate incentive for Barry to push beyond his limits. It's win-win for Zoom.
1 I mean, it sort of is, but there's an intermediate step that's rather important
2 Since the title of the episode is "Welcome to Earth-2", I spoil this with zero shame. I will, however, try to avoid some of the other spoilers from the episodes