I don't think it's unreasonable, BUT it does seem heavy handed. The adult equivalent of not getting your way so you take your ball and go home. It seems a bit controlling and dictatorial as well. While it's well within your rights, is it more or less likely to make her want to reconcile?
I"m not saying you should continue to pay for her lease, but maybe the conversation could have gone like:
You: "Now that we're separated, I don't think it's fair that I keep making lease payments on a car that I'm never going to use. What do you think about that?"
Her: "Hmmmm. I see what you mean."
You: "Why don't I take the car and continue to make the payments, and that way you can buy something that works for you."
Her: "That sounds reasonable."
Honestly, it probably wouldn't work like that. Her initial response would probably be more like "F%$K YOU!", but after she had time to think about it a little while, she'd probably come around, and appreciate that you were attempting to work with her, or at the very least, not resent the $hit out of you for being dictatorial and taking her car.
M:23 T:26 Me:53, Wife: 60 S:18 D:16 filed 7/16 W moved out 4/28/17