How to reconcile Matthew 5:22 and 1 Corinthians 15:35?
First of all, the comparison, in the question, should have been between Matthew 5:22 and 1 Corinthians 15:36, just to be strictly correct here.
Although both English translations, when translating the Greek have come up with the word 'Fool', in the two verses in question, there are two different words in the Greek transcripts, portraying two different meanings.
In Matt 5:22, the harsher Greek adjective, 'more' when transliterating, is used, which according to Strong's means 'moron'(mentally inert), among other harsher meanings, whereas in 1 Corinthians 15:36, the milder Greek adjective, 'aphron' is utilized, meaning 'lacking true moderation, lacking perspective', to put it simply. Consequently, Paul, in using the milder adjective is not necessarily, if at all, subject to 'Gehenna', which, after all, is a derivative of a fiery hell, an implication given by Jesus Christ himself Matt 5:22; 18:9; Mark 9:47,48, which would be harsh judgement indeed. Also, in the first instance, the adjective was used, by no less than the son of God himself, in reference to one's brother, to whom Jesus says one should be more tolerant, whereas Paul was not talking specifically about anyone in general.
FOOL. rather than denoting a person who is lacking in mental ability, the word "fool," as used in the Bible, generally refers to an individual who spurns reason and follows a morally insensible course out of harmony with God's righteous standards. Various Hebrew terms denoting such a one are kesil' ('stupid one'; Pr 1:22), 'ewil' ("foolish one"; Pr 12:15), na.val' ('senseless one'; Pr 17:7), and lets ("ridiculer"; Pr 13:1)). The Greek a'phron refers to an "unreasonable one" (Lu 12:20), a.no'e.tos to one "senseless" (Ga 3:1), and mo.ros' to a 'fool' or "foolish one" (Matt 23:17; 25:2)....
...Jesus Christ rightly referred to the Scribes and Pharisees as "fools and blind ones," that is, persons lacking wisdom and being morally worthless, for they had distorted the truth by manmade traditions and followed a hypocritical course. Moreover, Jesus backed up the correctness of this designation by illustrating their lack of discernment. (Matt 23:15-22; 15:3). However, the individual wrongly calling a brother a "despicable fool," judging and condemning his brother as being morally worthless, would make himself liable to Gehenna. - Matt 5:22; Ro 14:10-12; Matt 7:1,2. This reflection on the word "FOOL", is taken from "Insight on the scriptures", Vol 1, page 846