ぼく の ほう から ちかよる き が なくても、きみ に ぶつかられたら、ぼく は こなごな に なってしまうん だから.
What do the words in bold mean? I understand most of the text but can't find the meaning of き and the role of ん in the sentence.
the word "ki" here roughly means "intention" or "interest".
き が なくても would thus translate to "even if you don't intend to..." or "even if it's not your intention to..."
and the ん, as @Breton Loïc stated, is a simplification of の. If you look elsewhere in stackexchange, you will find more detail on its use.