Comte de Guiche is a lustful, manipulative, and vengeful count who stops at nothing to get what he wants. His pride and wealth allow him to spend exorbitant amounts trying to assassinate those who dare to insult him, and his position as the Cardinal’s nephew enables such behavior. De Guiche is also marked by his cowardice – his unwillingness to face Cyrano in direct combat, his infamous removal of the white plume during the siege, and his reluctance to join his troops in battle. But his most consistent characteristic is his lust for revenge. Every time he is insulted, even in the smallest of ways, he goes for revenge; and the punishment very rarely fits the crime.
On top of all that, he’s a total jerk to Roxane. FIRST his plan is to force her to marry Vicomte de Valvert, his underling. When the opportunity arises, however, he decides to take her for himself, even though he’s married. Worse, he decides to take her for himself in a convent, which means he’s perverting religion as well. (Cyrano even goes to so far as to call him "the devil," which fits our evil theory nicely.) On top of that, Rostand makes it pretty clear that this is a not-so-nice guy with his clothing and appearance. In Act II, he comes for Roxane dressed all in black – with a mask and a cloak, which is about as close as you can get to a name tag that says "Bad Guy #1."
If one wants to play devil’s advocate, though, one might point out that de Guiche, just like Christian and Cyrano, is simply vying for Roxane’s love (although to do so one would have to ignore the married part, so maybe that isn’t so valid after all). But, one could also point out that Cyrano challenges everyone and their mother to a duel after he gets insulted, so why should the Comte be expected to act any differently? Cyrano kills 100 men between Act I and Act II – what’s wrong with de Guiche killing one man (Cyrano) in Act V? (We know, we know, this is subject to debate, but just hold your horses for a second.)
De Guiche is made a villain not for killing Cyrano, but for the way in which he kills Cyrano. He doesn’t challenge him to open combat or have a swordfight – he hires a lackey to drop a log on the man’s head. Hardly noble. That’s like the whoopee-cushion of murder tactics.
Of course, it’s not exactly clear whether de Guiche is responsible for Cyrano’s death. Our previous argument supposes that it is, but if you were feeling particularly humanistic, you could probably make the case that de Guiche reforms by the end of the play. In Act IV, he volunteers to stay and fight with the men, though this likely means death. Cyrano entrusts him with Roxane, and he doesn’t violate this trust as he safely escorts her back home. In Act V, he apologizes to Roxane and earns her forgiveness. If this is the case, then his "warning" to Le Bret actually is, in fact, a warning.