As I understand it, most capitalists would argue that the reason the rich deserve their wealth is that they have worked for it,

One of the reasons, correct.

and the fact that they have managed to gain such wealth shows that they will do good things with it that will benefit everybody else.

Nope, I've never heard that argument.

which would help to ensure that people have worked for what they have

well, yes

and therefore have proven that they can use their wealth well

as said, no one argues that.

What you are getting mixed up with is the position that not letting people have keep their wealth is a disincentive to wealth creation

This is actually something most* people agree with, with the arguments being at which point the effect kicks in, and how strongly.

*communists and libertarians aside

As I understand it, most capitalists would argue that the reason the rich deserve their wealth is that they have worked for it,

One of the reasons, correct.

and the fact that they have managed to gain such wealth shows that they will do good things with it that will benefit everybody else.

Nope, I've never heard that argument.

which would help to ensure that people have worked for what they have

well, yes

and therefore have proven that they can use their wealth well

as said, no one argues that.

What you are getting mixed up with is the position that not letting people have keep their wealth is a disincentive to wealth creation

This is actually something most* people agree with, with the arguments being at which point the effect kicks in, and how strongly**.

*communists aside

** right-wing libertarians aside, as they tend to believe it starts kicking in straight away.