Alex never wanted to kill Shushant; if he did, he would've slashed the guy's neck instead of his arms.
His intention was to decapitate the guy and then observe him.
Alex wanted to know what kind of penalty he was going to face.
Shushant had failed to fulfill the required condition of providing the system with 10 coins, so as stated by the system, he was going to face the penalty, and Alex just wanted to know what it was.
We can never be sure when the system will do something like this again or if everyone will be able to do it or not.
Alex himself wasn't sure if he would always have 10 coins left in his stock. What if he had an emergency situation and had to use every single coin he had on him?
Wouldn't he be in a situation the same as Shushant at that time?
So, he wanted to be aware of what he would face and what the consequences of not being able to complete the requirements were, but alas...