Belle already left Rumple three times and each time he let her go without a fight. He even said on several occasions, the latest being in Neverland, that he expects that one day Belle will see what a monster he is and leave him. So, no, I don't think Rumple will do anything to Belle if she leaves him. He even gifted Belle a library and an apartment after she left him. There is nothing in that scene to suggest Rumple had an ulterior motive for doing that.
Rumple didn't kill Milah because she left him or Bae. What caused him to snap was when Milah screamed at him that she never loved him. We saw in Manhattan that wasn't true. They were a young couple very much in love before Rumple went off to war. Milah's life wasn't in danger before she stupidly tried to rewrite their history together to get that jab in that would cut him to the quick.
The idea that Rumple would kill any woman who tried to leave him is not supported by canon.