Brimmer was an unscrupulous person. He was virtually blackmailing people that he investigated. Plus he had a horrible temper. Not quite a cold-blooded murderer but at least not a sympathetic character.

First impression makes the Brimmer murder look almost accidental, but it wasn't as much of an accident as it appeared. He kicked the door shut, grabbed her very aggressively and said "I can't let you do that" then slammed her into the wall, when she pushed him away he punched her in the face and she fell into the glass table. Whether he lost his temper or not he was not going to let her ruin him.

Had she not fallen and died what could he do next anyway, once he grabbed her and punched her it was way to late to fix anything with her, his only choice would have been to kill her or have his life ruined.

Yeah, that's a really good point. Although it was an accident, if he had roughed her up, threatened her and then let her go, she'd be even more inclined to ruin his life and business through her husband.