He didn't stay dead...

Was there any legal reason why Jesus should have stayed dead?

YES.
Because our sins had been placed upon him. Although he wasn't a sinner on his own, he died covered with sin. Carrying our guilt. Hanging on that cross he was a wretched miserable filthy wreck...because that's what we are... and that's what was lifted from us and placed onto him.

BUT HE DIDNT STAY DEAD.
So what does that mean...? It means you are forgiven. It is a representation of God's thinking towards you. If your guilt was placed on to Jesus Christ, and then he took it into judgment, and then He came back out safely again, that is supposed to show you that your sin is forgiven, through Him.

HE WAS CARRYING YOURS, AND HE MADE IT.
THIS MEANS YOU WILL MAKE IT.
Death is only a penalty for sin--that's all it is. Jesus Christ carried your sins, and then passed through death safely. This is a picture for you, indicating that you will pass through death safely...IF you are with Him.

BELIEVE and FOLLOW the God-man who takes responsibility for all of your bad choices, failures and oversights. The One who went through Hell for you.

"For God loved the world so much that He gave His only begotten Son, that whoever believes in Him should not perish but shall have everlasting life!
...whoever believes in Him is not condemned. But whoever does not believe is condemned already..." John 3:16+