why did they do this and when did they start doing this? or did they do this at all? I had never head of this before but this Wikipedia page says they did it before 1603 https://en.wikipedia.org/wiki/Mustafa_I. also why did they stop after 1603?
The concept of royal fratricide was introduced and made law by Mehmed II although practice sometimes happened before him, especially with Mehmed I, who fought several of his brothers to secure the throne. The purpose was largely there to stop the empire of descending into civil war or minor succession crisis that would erupt in and following succession. When a sultan succeeded to the throne, he would imprison his brothers, and when his first male heir was born, he would strangulate his brothers and other male heirs with a silk cord.
The practice grew unpopular as time passed and in 1603 was replaced with the Kafes(literally cage) system, where heirs were kept inside the palace in a system of constant surveillance. Needless to say this created quite a few insane Padishahs.