That's irrelevant.
If Islam is true, then Christianity (with its Trinitarian system and a crucified God) *is* evil. Most Muslims wouldn't disagree with that. (By "most" I mean the statistical average worldwide, not "most" of the Muslims presented on TV, or the ones invited to speak at Princeton, or proselytizing in America for a friendlier face of Islam).
That fact does nothing to undercut the argument that Islam is evil. If the Christian worldview is correct, Muslims are erecting a false religion which suppresses the truth of God. Thus, Islam is evil in the Christian worldview. If Islam is true, Christians are asserting blasphemous things about God. Thus, Christianity is evil in the Islamic worldview.
I don't see what you think the problem is, unless you want us to answer the question from a secular worldview (in which case there is no evil).