To say that Matthew 15:4 proves that Jesus supported capital punishment is to ignore the context of that chapter. He wasn't affirming the death penalty. He was quoting the Mosaic law to show the inconsistency and hypocrisy of the Pharisees.

Taking an isolated snippet out of Scripture is easy and anybody can do it to "prove" anything they like. But the preponderance of what Jesus said and taught in the Gospels clearly showed an ethic of mercy. It is unmistakable for anybody willing to take an honest look at the whole of the Gospels.

But, since you claim that Matthew 15:4 is clear teaching on Jesus' part, let me ask you this: Do you believe that we ought to put people who dishonor their parents to death? If you don't, then isn't it inconsistent to use that passage to try to support capital punishment for reasons Jesus didn't name?