Verified answer

な (na) is a particle with three main uses:

(1) At the end of a sentence, na is used (mainly by men) to show emotion or emphasis:

うまいな！｜This is great!

これはだな，こうやるんだ｜This? Here's how you do it.

お前は本当に知らないんだな｜You really don't know, do you.

よくもここへ来られたな｜What nerve you have to come here!

(2) When used after a -masu stem of a verb, it is a command. This is an abbreviation of the command form - nasai:

早くしな｜Be quick.

これ1ダースくださいな｜I'd like a dozen of these.

(3) However, when used after the dictionary form of the verb, it means the opposite - _don't_ do this. So, it's often used by adults (parents, teachers, etc.) to children:

このドアを開けるな｜You mustn't [must never] open this door.

芝生には入るな｜Keep off the grass.

あの人に心配するなと言ってやれ｜Tell him not to worry.

手紙を投函するのを忘れるなよ｜Don't forget to mail the letter.

Na has a different use when following a na-adjective or noun. Here, it only serves a grammatical purpose, to connect the adjective or noun to something, usually another noun:

有名なところです。｜A famous place.

学生なので、行かない。｜Because he's a student, he won't go.

Matthew explained it perfectly. In the example you provided, 'na' is used on the end of a word to make it into an adjective that better accompanies the word that follows, if that makes sense.

For example:

しずか　＝ [ shizuka ]. It means 'quiet'.

しずかな＝ [ shizuka na ]. It means 'quietly'.

In your songs, [ kirai ] means 'hate'. And [ boku ] means 'me'.

To answer your question properly, I'd have to hear the context of the phrase 'kirai na boku'. But it means something along the lines of 'hateful me', so basically meaning that they did something to deserve being hated, they are able to be hated. Does that make sense?