Why would the apostles allow themselves to be martyred in such horrible ways for saying Jesus existed when they knew that was false?
Instead, they said they were eyewitnesses of Jesus' miracles and resurrection. And they all were martyred separate from each other.
It could be said that one of them was crazy and suicidal...but it would be hard to make the case that ALL of them were (except John, who wasn't martyred but was exiled to the island of Patmos instead).