Aggresion

My 3 year old son has began acting aggresive (aggressive) and hateful when I was pregnant, this seemed to get better while I was on maternity leave (baby now 4 months old). He has been back in daycare for a month and the issues seem to be rising again.
He will hit other kids in the face, hit me if I discipline him, talks mean (told a friend he was going to kill him - never before has said this!). He will tell us he wants us to leave, is VERY demanding when he wants things, will scream demands.
We are concerned his childcare environment is causing him to become hateful. He is a very sweet child 90% of the time, is VERY good to his baby sister and is loving.
We are starting to become concerned at his behavior and wonder if we need to have him see a professional to get tabs on his behavior. While I was pregnant, his daycare teacher told us he was aggressive yet the other kids in class were also but we were concerned the upcoming birth of his sister was causing him to act out for more attention.
We are not sure what to do at this point. My mother in law is a school Psychologist and has not expressioned concern - yet.

[Kevin Kennedy, Ph.D.]

There's no sound reason to avoid arranging an evaluation by a child psychologist. In other words, you can only help yourself by such a decision. One way or the other, you must manage the behavior when it occurs. The intervention itself is simple. At the least sign of aggressive behavior, place him in time out (adult-size chair) for five minutes. Use a self-standing (not microwave clock or wall clock) cooking timer to track the time. Start the timer only when (a) he is seated in the chair and (b) he is quiet. If he makes any noise during the five-minute period, or is he leaves the chair, start the time over.