I understand that God sometimes will put on some testing on us to test our faith, but how does it justify if in the process involves someone's passing away?

Not knowing the circumstances around your question, I will try to share some thoughts. If the person who passed away was a Christian, then he/she has gone to a better place and if you are a Christian you will see that person one day. I assume that the person was very dear to you. The best thing to do is to seek God's face and His love. As you do this, you will develop a closer relationship to God the Father and our Lord Jesus Christ. The pain of the loss will never completely go away. But God is always there to put His arms around you and let you cry on His shoulder and comfort you. In time, as you continually seek to develop a deep relationship with God, He may reveal the answer you seek concerning this test. May God's love surround you and may His presence continually be felt by you.

God Bless you.
Yes sometimes this is the case. If you read the book of Job. Job was tested by the Lord, He lost all of his children when their house burned to the ground.
The Lord tested Job for many years.
We must have faith in the Lord and continue to walk in the light ... walk the talk as my pastor used to say and keep your eyes fixed on the cross.
Sad worldly lossed will be replaced by the Glory of God and the Kingdom of heaven.
Amen