A lot of really well regarded philosophers throughout the ages have disagreed with that.
> I guess my point is that taken less cynically, rights... only exist [with] sufficient power
If I follow that line of thinking, justice is only an extension of power. I don't think people agree with that.
I don't believe you think that. Perhaps it's just a matter of phrasing; surely we both agree that it's important to recognize that people should never murder people. We use the word "right" because it's important to be absolute in this rule.
It's worth distinguishing between:
* life is not a right
* we don't have enough power to enforce murder laws
tl;dr To say there are no absolute rights is to say there are no absolute wrongs.
There aren't any absolute rights or wrongs, there aren't any absolute morals at all; right and wrong is always a point of view and no one can lay claim to having the absolute and only correct ones.
You are confusing legality with morality. There isn't necessarily any connection.