In the case of Richard Dawkins - really, he's a gentleman by the standards of debate on almost any other topic. At times he is passionate or mocking, but aren't we all when faced by manifest injustice, nastiness, or cruelty? In his book, The God Delusion, taken as a whole, he does indeed argue for the unpopular point of view that we should be emphasising the ways in which ordinary religion resembles delusion, rather than whatever ways they are distinguishable, but the general tone is very mild. At most, it is rather satirical (the paragraphs are laced with humour throughout - though there is also plenty of thoughtful discussion, with concessions and qualifications as needed).
In public appearances, Dawkins is usually gentle and tolerant - far more so than most people speaking on most subjects that they are passionate about. I've seen him go out of his way to be courteous to annoying and ignorant interlocutors.
As one of the commentators - "ben" - at the above link, says...
The religious zealots have adopted a very clever tactic, namely branding anyone who speaks out against religion as "strident", "shrill"or "angry" in attempt to plant the idea that atheists are persecuting them. The problem for them is that religion has had a free ride until recent years and any questioning of their superstitions is therefore seen as aggressive when in fact it is no more so than in any democratic debate. I for one do not intend to hold back because this is precisely what they want.This is exactly right: there has been a largely successful attempt to hold atheists who actually argue publicly for atheism to a special standard. Anyone who does not meet it is thereupon demonised, or simply dismissed as "shrill", "strident", etc.
That's not to deny that there can be genuinely uncivil and over-the-top language from atheists. There can be, as from anyone else on any other topic. But it doesn't come from Richard Dawkins, and when we do see examples of it they are nothing compared to the sort of language that is used by people who are debating politics. With the Republican primaries going on in the US at the moment, that country is currently engaged in forthright political debate - and the level of incivility on all sides is at least as bad as I see from anyone debating the truth or falsity of religion.
Meanwhile, forthright Christians who want to argue publicly for the truth of their views manage to be at least as "shrill" as Dawkins. For example, I'm currently reading Mere Christianity by C.S. Lewis, which I don't think I've previously read in its entirety and have not opened for decades. As usual with Lewis, his style varies between blunt, emotive, self-righteous, and downright snide (I'll bracket off how naive the actual arguments are). His approach gets a free pass in our culture, but if an atheist wrote in exactly the same way he or she would be roundly condemned.