The angels (that already have full knowledge) gain what exactly, by observing humans die in a way other humans aren’t even aware of? Why would any god want to test angels?
Angels have lesser capacities in some way than men, though they are bestowed with stronger powers in the capacities they do have. From the bible, an angel can be all good or all bad, but never redeemed or saved. That is why angels long to look into the redemption of mankind. It is an exotic sight to them.
Angels do not have full knowledge, but have wonderment at God's workings, while they were tested as they are free moral agents.
The devil as the first rebel against valid authority engaged in the first "occupy movement," that of the throne of God, ("I will be like the Most High"), making the first "share the wealth" demand, that God share His power and omniscience, working to persuade Eve that she was a victim of Divine selfishness (God's law is preventing you from being like Him), and had a welfare right to what was not hers (victim-entitlement mentality).
The devil, acting as one embittered by unjust treatment, seeks to persuade others to adopt a victim-entitlement mentality and rail against the God of the Bible, and or worship false gods that are in fact a distortion of Him and His justice and mercy which find their reconciliation in the cross of Christ.
In contrast to the selfish lust of the devil, in God's kingdom, in addition to salvation by unmerited grace, rewards are earned (by grace), so that he/she that overcometh him will Christ grant to sit with Him in His throne," (Revelation 3:21) and shall know as even also they are known. (cf. 1Cor. 13:12; 1Jn. 3:2)
Now you know where the liberal ethos comes from, as well as the atheist's rage against God.