Snape's motivation to do unbreakable vow

Why would Snape do the unbreakable vow? What was his motivation?
He couldn't have been doing it to prove anything to Voldemort because he wouldn't want Voldemort to know about it.
And I don't see how it could prove anything to Dumbledore either. Therefore he could only be doing it for himself. What is his motivation though?

Did he really know what the plan was? Or was he being a very good con? Could he have been doing unspoken magic on Bellatrix? An Imperius and not an actual unbreakable vow?
I know it's a stretch, but it's an idea.

Because slimy gits need love, too.
I'll go and dig it out.
Then I'll get that drink.

Boy you're thirsty aint ya?
DO we need an intervention here?

Thats an interesting theory but maybe Snape made the unbreakable vow because he cares about Malfoy and his mom and doesnt want anything bad to happen to them. Plus im sure Dumbledoor was made aware of the vow and he would have made sure that he had a secrate plan to avoid death if he really wanted to.

well this would ensure that he had to do what DD asked him. This way he couldnt back out or he would die