Ni particle in complex sentence

Question

暴力に正当な理由があるとすれば、自分や誰かを危険から守るためだけだ。 in this sentence why did we use に particle after 暴力. how does it come here, explain the grammar

Answer 1

That is a complex sentence, but actually, only the first part is relevant.

Maybe the size of the sentence is a little distracting.

「暴力に正当な理由がある」

「ぼうりょくにせいとうなりゆうがある」

There is a justified reason for the violence.

Basically, に is there because of ある。

I googled 理由がある。

A book on Amazon...

不愉快なことには理由がある

A book defending jazz...

すごいジャズには理由（ワケ）がある

An page about energy bars...

その栄養には、理由がある。

It's kind of like English.

"There's a reason to..." or "There's a reason in..."

You can read more about に and the verbs that like it at this question.