Why does Bentham say that ‘all punishment in itself is evil’ in the third paragraph?What is the main function of punishment

Turn to Reading 7, The Purpose of Punishment, in Part IX of the set
book and read the first few paragraphs, from the beginning on p. 580
until the break on p. 581 (up to the paragraph ending ‘… as cheap a rate
as possible). This is a combination of extracts from Chapter 13 of
Benthams Principles.

1 Why does Bentham say that all punishment in itself is evil in the
third paragraph?
2 What is the main function of punishment (see the third paragraph of

p.
581)?
3 Why do you think Bentham says (on p. 580, fourth paragraph) that we
should not punish someone if the charge against them is groundless?