So, let's look at it again.
This is tiresome, but okay.
Notice that the "whosoever" in the second phrase is not limited to the "they" in the first phrase.
So again, the question, could some that professed to being Christians have been in that category?
What is the point of asking, if you profess not to know if any professed Christians were actually in that category???
All sorts of things are hypothetically possible in the abstract. John 16:2 may allow for the possibility that professing Christians might kill apostles, but it isn't evidence that this happened. The verse doesn't say who the "whosoever" would be. It doesn't preclude the abstract hypothetical possibility that the apostles were killed by time-traveling Romulans, but I doubt it.
As a simple matter of fact, we have absolutely no record of any professing Christians killing any of the apostles. James the son of Zebedee was executed by order of Herod Agrippa I, who made no pretense of being a Christian. James the Lord's brother was thrown down from the temple and beaten to death by Jewish, non-Christian opponents. Peter and Paul were executed by order of Nero; I'm pretty sure he didn't claim to be a Christian, either. Fairly reliable traditions indicate that Andrew and Simon the Zealot were also crucified. We have little or no reliable information about the deaths of the other apostles, but the traditions about their deaths say nothing about other Christians, real or professed, murdering them. For a judicious and brief review of the deaths of the apostles, see the following PDF document:
With regard to the expression "not sparing the flock" in Acts 20:29, you asked a lot of rhetorical questions but didn't explain its relevance to the question about the deaths of the apostles. I understand that you interpret this expression to refer to a complete, total apostasy. I don't, but let that pass: the text does not indicate how the apostles would die, let alone who would kill them. It doesn't say anything about the apostles' death! So I still fail to see the relevance of that text to our discussion here.
Finally, you wrote:
No, I don't. Protestantism does not teach that the church ceased to exist on the earth. Rather, it teaches that the church was corrupted and needed to be reformed. What I do find ironic is that you don't seem to know this.