I previously answered a similar question about Arnold's motivations for switching sides in the War of Independence in an earlier post u/enygma9753.

He had plenty of financial and personal motives, coupled with petty resentments, real or imagined professional slights and blatant opportunism, explained in the link above. He claimed he had principled reasons for turning, but these mirrored common Loyalist ones at the time.

Also, the revolution's outcome was not a foregone conclusion. He had been defeated by the British in the failed siege of Quebec in 1775 and later injured at Saratoga, so he knew firsthand how resolute the British were, with experienced and capable commanders. Until 1778, the British had the upper hand in most engagements. He was no doubt aware of the shifting tides of war when France entered the fray, but such factors didn't outweigh his own motives, which were paramount to him.

He was easily offended and had a knack for earning animosity in both American and British ranks.