Could be various reasons:

- evilness (hate)
- childishness
- revenge (i.e. an atheist comes to revenge in words against an article that speaks against atheism)

Matthew 5:10-12

10 Blessed are they which are persecuted for righteousness' sake: for theirs is the kingdom of heaven.
11 Blessed are ye, when men shall revile you, and persecute you, and shall say all manner of evil against you falsely, for my sake.
12 Rejoice, and be exceeding glad: for great is your reward in heaven: for so persecuted they the prophets which were before you.