Why is the symbol of Jesus the cross when that was what he died on?

Great question!  It seems odd, doesn’t it?  No one today would choose an electric chair, or a hangman’s noose, to symbolise their religion or to represent their leader! But that’s what the cross was, the means of torture and execution that Jesus died on.  It’s because Christians understand Jesus’ death not as something terrible that happened to him, that cut short his life, or interrupted his work, but as the very thing that he came to do. This is how Jesus understood his death, and he predicted his own death many times.  One example is John 10:17 - 18 “I lay down my life—only to take it up again. No one takes it from me, but I lay it down of my own accord.

The Bible says it’s because of Jesus’ death that we can be forgiven for ignoring and rejecting God. He took the death and separation from God that we deserve.  So the death of Jesus was horrific, but it was part of God’s plan, and the way that we can be in right relationship with God.  It’s the same reason that Good Friday can be considered “good.” Evil people did terrible things, but God uses them for our good.