Because sin is not merely bad actions, but an attitude of rebellion against God.
One part of the answer then is that sin in an infinite offense against an infinite God.
But I don't think that on its own is enough. Another part of the answer is that the biblical pictures of punishment show that even while being punished, sinners continue to curse God (eg Rev 16:9, 11, 21).
The Christian answer is that the evidence is God coming to earth in the person of Jesus - dying for the sins of the world and being raised to new life as recorded in the Scriptures. Claims about the personality of God such as "love" and are backed up by the historical events of the crucifiction and resurrection (eg John 3:16 - God so loved the world that he gave his only Son).
Now maybe you don't accept that evidence. Fair enough. But in order to fathom why Christians believe that God has certain personality traits, the evidence that we are basing our understanding on is the person and work of Jesus as revealed in the Scriptures.
"The algorithm to do that is extremely nasty. You might want to mug someone with it." -- M. Devine, Computer Science 340