Here are my thoughts; again, Cliffs are provided at the end.
No, I would not be able to defend that claim, as it is my opinion that there is presently insufficent evidence to support such a claim. Of course, there are plenty of studies that demonstrate an increase in particular symptomatology in association with the initiation of a psychotropic agent. For instance, there is an increased likelihood of seizure in those initiating Wellbutrin, an antidepressant medication. That increased likelihood, while statistically significant, is still clinically unlikely (i.e., while, statistically, you are much more likely to have a seizure when on Wellbutrin than when not on it, even when you are on Wellbutrin it's still highly unlikely that you'll have a seizure).
The issue implied here is whether or not the Lexapro had some causal effect in him doing what he did - predatorily murdering, sexually assaulting, and cannibalizing a girl. Regardless of whether or not Lexapro is associated with an increased risk of experiencing sexually explicit, violent fantasies, I can assert with confidence that there is absolutely no evidence to date to definitively support the notion that Lexapro causes irresistible urges to engage in sadistic, sexually assaultive, cannibalistic behaviors. If there were, then I would almost guarantee that his legal team would have entered a plea of not guilty by reason of insanity (NGRI) or perhaps some defense related to diminished capacity (i.e., they would have said, "Jury people, yes, our guy did it, but he really didn't know what he was doing was wrong because he was on this drug - Lexapro - that makes you do crazy things"). Why didn't they do it? Case and point: there is no evidence to support such a claim.
What his legal team will now try to do is claim that the Lexapro constitutes a mitigating factor for the purposes of attempting to avoid the death penalty. In short, it works like this: if the defense can prove that there is a mitigating factor present, with a mitigating factor being anything that may have made it more likely for him to have done what he did (regardless of how seemingly insignificant that thing may be or how silly it may seem to you or I), then, by law, he cannot be sentenced to death. So, at this point, his legal team will bring in all manner of experts to testify about all different sorts of things - how he may have had a crappy childhood, how he was on such-and-such medication, how he thinks the color blue makes him look fat, and on and on ad nauseum - in a last ditch effort to convince the jury that he doesn't deserve to die. In my opinion, it won't much matter who they call or what they say: this jury will likely be swift in delivering a death penalty verdict.
Cliffs: The Lexapro claim isn't supported by the available evidence, but that won't stop his legal team from asserting it as a mitigating factor. In the end, though, it won't matter, as this guy looks to be on the fast track to the electric chair.