PR doesn’t have negative or oppositive voting
If a system qualifies as PR, it won't have mechanisms to do either of the following:
1) Vote in an effective way against a specific candidate
2) Vote in a way that precisely cancels out someone else’s vote
I don’t have a rigorous proof for the above statement, but here’s the basic idea: In the platonic ideal of PR, every voter wins. Every voter is represented. Real PR systems approximate this ideal. In such a system, the ability to neutralize someone else’s ballot with your own would be like saying, “My faction is willing to go unrepresented if in the process we can take out that other faction with the same number of people. We’ll give up our place at the table as the cost of denying them a place as well.” But PR doesn’t work like that.
And it shouldn’t. Suppose there’s a particular idea you don’t want represented in a legislature -- like climate change denial, which is rampant in the current US congress. You don’t get rid of the idea by denying representation to the people who hold it. You get rid of the idea (or in any event reduce its clout) by somehow educating people to hold better ideas.
“Darkness cannot drive out darkness: only light can do that. Hate cannot drive out hate: only love can do that.” - Martin Luther King, Jr.
In contrast to PR, a winner-take-all system might have the features listed above. That’s because in winner-take-all it’s acceptable for large numbers of voters to lose. That’s how it works.
The PR ideal is that all the voters win. All the voters are represented. As described above, letting one person cancel out another’s representation makes no sense.
The winner-take-all ideal is that all the voters have a fair shot at winning, a fair shot at being represented. These aspirations are low enough that letting one person cancel out another’s vote doesn’t necessarily make the system any more unfair than it would have been otherwise. It doesn’t automatically break the system.