I will attempt to make your argument more concise:
P1: Free will exists.
P2: The theory of infinite universes is true.
C1: (From P2) Therefore, there exist an infinite number of unique universes.
C2: (From C1) Therefore, in at least one of those universes I must freely commit a specific self-deprecating irrational action (e.g. cutting my leg off).
C3: (From P1 & C2) If free will exists, no agent can freely choose to commit an irrational self-deprecating action. (I can only assume this is what you assumed to invoke a contradiction). Therefore, either free will or the infinite universe theory is false.
C: The infinite universe theory is false.
What is required for the argument to work?
Assuming the premises are true.
Supposing the infinite universe theory entails every possible event happening.
Taking those as true, P1, P2, C1 and C2 are true.
But, the leap required from these to C3 is just inexplicable to me. Furthermore, it seems to me when you arrive at the desired contradiction, you randomly choose which premise is false (or maybe you have already concluded it is true).
So, I'd say even though I thoroughly disbelieve the infinite universe theory, it cannot be disproven by the existence of free will.
An aside note:
I was about to comment on the "large infinity" and similar concepts (found in other answers) by declaring them irrational, instead, here's the example which helped me grasp it.
Premise 1: An infinite number of flavours exist.
Premise 2: An infinite number of ice creams exist.
Conclusion: There exists at least one ice cream of each flavour.
The conclusion obviously cannot be determined from the premises. Since there could be an infinite number of ice creams of the same flavour.
So, an infinite number of worlds/universes doesn't guarantee all possible features.
Note: if the requirement is that all worlds (ice creams) are unique that seems to me to guarantee all possible features but I am not sure.
P.S. _I'm new here, don't know if it's acceptable or productive to revive old posts like this. Would appreciate feedback. Thx.