Is it permissible to grieve for a non-Muslim oppressor and murderer because he died in a state of disbelief, and does this contradict the creed or is it considered a trial for the heart?

It is permissible to grieve over the death of disbelievers, out of compassion for them due to the torment of Hellfire and eternal damnation that awaits them. This is a permissible form of mercy, just as the Prophet, peace and blessings be upon him, wept at his mother's grave. This falls under the general mercy for all creation, both Muslims and disbelievers. Ibn Taymiyyah said concerning the people of innovation: "If you look at them with the eye of divine decree, you will pity them because confusion has overtaken them and Satan has overpowered them." Ibn al-Qayyim said: "Had your Lord willed, you would have been like them, for the heart is between the fingers of the Most Merciful."