"I'm already in trouble, I might as well go further" or "I'm already in trouble, I might as well make the punishment worth it." What phrase refers to the state of mind of a person who already believes themselves to be in so much trouble that they might as well continue doing the act that gets them in trouble because it can't get any better?
I've also seen the one that Cugel mentioned; that's close to what you're describing, though it's more about the risk than the actual outcome. (Which is to say that you may not be hanged at all; you're merely risking that possibility, whereas you're seeking something where the person believes that things cannot get better.)
The one I've seen used commonly, which is also imperfect for the same reason, is "in for a penny, in for a pound". It has a broader usage than the one that you describe, in that it also refers to the amount of effort that someone applies to something regardless of whether they're in trouble. However it can also apply to situations where the speaker has risked the loss of the proverbial penny, and may as well take the additional risk of losing the proverbial pound. But again the problem is that the speaker doesn't actually believe that they WILL lose the pound. Still, both are somewhat close to the concept.
One might as well be hanged for a sheep as a lamb
which is common in the UK.
In the bad old days, if you were caught poaching on the master's land, you got the death penalty regardless of what you were poaching. Hence, if you were going to poach at all, you might as well go for the bigger, more valuable "sheep," as opposed to the smaller "lamb." Or, indeed, two sheep...
Phrases such as go the whole hog, go for broke, go all out or go all the way mean to do something in whole rather than in part, and particularly to act without restraint. For example, the Cambridge Idioms Dictionary defines:
go the whole hog (British, American & Australian) also go whole hog (American): to do something as completely as possible. It was going to cost so much to repair my computer, I thought I might as well go the whole hog and buy a new one. I went whole hog and had a huge steak and French fries.
They are often preceded by "might as well" to convey the sense you want.
And for a starter I would go to work and steal Jim out of slavery again; and if I could think up anything worse, I would do that, too; because as long as I was in, and in for good, I might as well go the whole hog. — Mark Twain, Adventures of Huckleberry Finn (1884)