ULTRON has been described as "child like, unstable, disturbed, psychotic, quippy" and that was the plan since I first read about Joss directing this movie. I think people want "evil villain" and if they are eccentric or not portraying pure evil in every scene they consider it a negative. ULTRON has traits of Tony Stark, so of course hes going to be humorous as well as deadly. But personality wise, I think hes all kinds of things at once.
ULTRON is an AI that becomes self aware and decides he needs to wipe out humanity to achieve true peace. Thats not good for humans BUT that doesnt mean hes suddenly just an "evil villain". To ULTRON, he is the hero of the story. Do you know what Im saying?