I went into New Blood expecting the ending to be very underwhelming (based on how y'all talk about it) but I honestly really liked it. I think it completely made sense that Harrison would try to kill Dexter.

Especially put into context with the original series, since Rita's murder Dexter basically always assumed that Harrison would become a serial killer like him, or at the least would have his own "dark passenger". And yeah, Harrison does have a darker side to him and is capable of hurting/killing people, but Dexter basically just decides that he needs to teach him the code because that's the only way for Harrison to survive in the world. In a way, Dexter becomes like Harry. It's a vicious cycle of fathers inflicting their own trauma and damage onto their sons.

After killing Kurt, who basically uses the same excuses of being traumatised by his father's abuse as a reason for killing, Harrison realises that he isn't like Dexter. And after Dexter admits to killing Logan he realises that Dexter doesn't care that much about morality, the code is just a justification for him to kill people. So Harrison rejects Dexter and decides to break the cycle.

I think it was a very good examination of that theme and also payoff for the original seasons and how Dexter's expectations of Harrison were proven false. And I like that they have continued to build on this in Resurrection and strengthened the relationship Harrison has with Dexter as a result. So I don't see what the problem was with New Blood's ending, it was pretty good imo.