Are both sentences OK?
First of all, I want to thank everybody who voted for me.
Whoever is responsible for this, come out.

Question

Answer

The first sentence is good.

In the second sentence, the meaning is unclear. "Whoever is responsible for this" is another way of saying "whoever it was that voted for me", but I am not sure what you mean by "come out".

Do you want them to let you know who they are? If so, you could say "Whoever is responsible for this, please let me know."

If not, please try to explain your second sentence in another way and I will try to help.

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