grammatical number agreement

Question

You can turn men into a serial killer by brainwashing them.

I use them and men and then use a serial killer, which is singular. Is this allowed in English or not?

Answer 1

Grammatically it is fine

You can turn oranges (plural) into juice (singular) by squeezing them.

You can turn planks of wood into a table by glueing them.

But in this case it isn't meaningful, since it means you can join several men to make one serial killer... and this is practically impossible.

Answer 2

Question: You can turn men into a serial killer by brainwashing them. [buzzer]

[I prefer to use examples that don't mention the word in the sample sentence].
Answer:

• You can turn men into frogs by enchanting them. [OK]
• You can turn a man into a frog by enchanting him. [OK]

If the noun is plural and the thing it is turned into is a countable noun, it would be plural. If the thing it is turned into is a non-countable noun, that's fine.

• You can turn aggressive dogs into gentle creatures by treating them well.
• You can turn coffee beans into coffee by grinding them. [coffee here is uncountable]