Life is a test and this world is a place of trial.

Ultimate justice is not possible without a second life.

Lets assume this life is not a test (when we die its over), and then have a look at the kind of life people (who according to Islam are the best creation of the Creator) here are leading:

The bad guys would win it all. They don't bother about being good and ethical in this life, live luxuriously on stolen money then just die and vanish. All the bad politicians, gang lords, serial killers, and so on. If all of those monsters got away with their bad stuff in this world and died, they've been very lucky. Which would be opposite of justice.

Same way, someone who born on streets, or born a cripple, or blind, or had the most miserable life ever till death. The ones who only saw troubles in this life and died the same way.

There has to be a second life, or else justice wouldn't be prevailed. If a 50 year old guy murders a 15 years old, no court of law can do the justice. At max they can hang the murderer. But then he already has lived his life for quite enough time, and the life taken had lots of it yet to come. Only a second life can do the true justice.