I dont think he necessarily hates him, just hates that he's inferior to him. like he said he has an inferiority complex that was "planted" by him. its like your significant other cheated on you so you hate on the person they cheated on you with but its not necessarily that you hate the person but you just hate that your lover cheated on you with them so you push that on to them instead of blaming your lover, you blame it on them. he was mad that he was always second place and his parents took that out on him so hatred grew in him and he just blamed it on the reiji for his inferiority complex. sorry for the long as paragraph LOL.
also very thin line between hate and love, sometimes that line is blurry, cliche ik lmaooo
The enemies to lovers trope of let's fuck just because reasons, has to be most bizarre irrational reason for two people to fuck, when if you truly hated someone fucking them would be last thing on your mind.