Indeed, those who have believed then disbelieved, then believed, then disbelieved, and then increased in disbelief - never will Allah forgive them, nor will He guide them to a way. (4:137)
If the punishment of apostasy was to kill the apostates, then how could they disbelieve, believe and disbelieve? It is fully clear and very easy to extrapolate that the ones who disbelieve are to be left alone and not killed so that they have a chance to decide if they want to believe again or increase their disbelief.
So making the claim that Islam teaches Muslims to kill apostates and that makes it a violent and unethical religion is completely false. I have seen many of Richard Dawkin’s videos, who is a confident atheist, and his biggest argument against Muslims is that we are taught to kill apostates. Well, here is the proof against it. That is all.