Is intentionally killing an innocent kitten sufficient for entering Hell, and does God forgive a sin of this magnitude?

What you have committed is forbidden, but repentance erases sins, no matter how great they are, even disbelief, as Allah Almighty says: "Say to those who disbelieve, if they cease (from disbelief), what has passed will be forgiven them," and His saying: "Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins." So, if you repent from killing the cat with remorse and a firm resolve not to return to such an act, then rejoice in Allah's forgiveness, and beware of despairing of Allah's mercy, as He Almighty said: "And who despairs of the mercy of his Lord except those astray?" Thus, repentance erases sin, no matter how great it is.