Since the man is involved in both (1) ball hits man and (2) man becomes blood, you should probably have the man detect the ball and add the blood to the world before removing himself.

You should try to code it yourself and then if you run into problems, post the code you tried and explain what is happening that shouldn't, and what should happen; AND, if you are getting error messages, copy/paste the complete message here and point out exactly where the error occurred or what line was highlighted.

public void die() { Actor Man = getOneObjectAtOffset(0, 0, Man.class); //this line is creating error if (Man != null) { getWorld().removeObject(Man); //how to replace this object with another one Greenfoot.playSound("aah.wav"); } }