A stupid question, but I am curious
 

Yesterday Ｉ came across this word: 竹刀. I had never seen it before, but I assumed it was a bamboo word. I look on the dictionary for it thinking it might be read something like ちくとう or (less probable) たけそり. With my amazement I found out it is read しない (none of the two a reading I knew for those 2 kanji).

Now, my question is this one. Is the reading しない because a bamboo sword is supposed not to cause death? (forgive me, but I had to ask).

You have such a a wild imagination.

The word comes from 撓い竹　＝　しないだけ = bending (or supple) bamboo.

The kanji 竹刀 is merely a substitute. We already had the verb "shinau = to bend, to be supple" before we had any contact with the Chinese.

Hi, chryuop.
Unfortunately, no.
There are many words like that. I mean many words we can’t tell the reading from each kanji.
For example:
東雲 You would read this “higashi gumo” or “tou un” but it’s “shinonome”.
We have many words like that and we just have to memorize them.
がんばってね！