Pensfan,
Damien was indigent. It does not surprise me that whoever prescribed the medication gave him a bad one. Indigent people are often misdiagnosed, given incorrect medication, etc. because the doctors treating them are often either inexperienced or incompetent, sometimes both. (BTW, if we don't get rid of Obamacare before it goes into effect, this could happen to any or most of us.)
None of the side effects you listed seem to indicate violent and/or murderous rage or actions. Damien said he suffered from some of the symptoms, but he also said that whenever he felt violent, he generally felt violent against himself, suicidal at times. And don't bring up that old "I'll scratch your eyes out" incident. Like so much in this case, that was a gross exaggeration.
As to Imipramine, I stated some time back that I believed that Damien was either over medicated or given the wrong medication. From what you have said, it seems that I was right. Again, none of Damien's problems are proof that he murdered Stevie, Michael and Chris.