Believing something is true does not mean that you should tell someone, especially someone in grief.
If someone’s husband just died, would you be willing to say, “You should know that he was cheating on you for years”?
If someone’s mother just died, would you be willing to say, “I really think you should have spent more time with her in her final days”?
If someone’s child commits suicide, would you be willing to say, “You could have prevented this if you had bothered to pay attention to the warning signs”?
Even if all these things are 100% true, I think it would be monstrous to blurt them out.
Sometimes compassion means respecting that people are not always ready to hear the truth.