Learn Turkish: Biz onu affedebildik; ona hiç beddua eder miyiz? - We were able to forgive him; would we ever curse him?

This is a rhetorical strong negation. By asking "would we ever do X?", you are strongly implying "we absolutely would not do X." In Turkish, pairing hiç (ever) with a positive aorist question like eder miyiz? (would we do?) is a very natural way to express that you would never even consider doing something.

Why do we say onu affedebildik but ona beddua eder miyiz? Both refer to "him".

It depends on what case the specific verb requires. Affetmek (to forgive) takes a direct object in the accusative case, so "him" becomes onu. However, beddua etmek (to curse) requires the dative case, because you are literally saying "to do a curse TO someone", so "him" must be ona.

I know the verb is affetmek, so why is it spelled with a d in affedebildik?

This is consonant mutation. When the ability suffix -(y)ebil is added to affet-, the vowel e causes the final voiceless t to soften into a voiced d. The exact same thing happens in a verb like gitmek when it becomes gidebilirim.

Is beddua a compound word?