He wants to give charity on behalf of some people because he feels guilty for falling short in his responsibilities towards them

Praise be to Allah.

Firstly:

If these mistakes were a kind of wrongdoing towards that person, then you must give them their dues or ask them to pardon you.

If this wrongdoing was financial, you must pay it back or pay its value. If it was something intangible, such as backbiting or telling lies about them, and the like, then you have to ask them to pardon you, in addition to praying for forgiveness for them and speaking well of them to others.

For example, you can do the righteous deed and give the reward thereof to the one whom you backbit or mistreated.

Ibn Baaz (may Allah have mercy on him) was asked about someone who gives charity on behalf of his mother when she is still alive; will the reward for the charity, whether it is in the form of money or otherwise, reach her?

He replied:

As for charity, it benefits both the living and the dead, according to the consensus of the Muslims. Similarly, supplication (du‘aa’) benefits both the living and the dead, according to the consensus of the Muslims."(Majmoo‘ Fataawa Ibn Baz 4/348).

See also the answer to question no. 65649 .

And Allah knows best.