In the penultimate episode of Fargo Season 3, Nikki Swango proves beyond a reasonable doubt that she is the smartest criminal in the season and possibly the entire history of the show. She outplays V. M. Vargas at his own game, walking away unscathed from a tense meeting in a hotel lobby that was supposed to end in her assassination. But Nikki can't do it alone, and employs the help of bus-seat sharer and new partner in crime Mr. Wrench to get the job done. Mr. Wrench and Swango seem to be fully healed since their bloody visit to a mysterious bowling alley, but it's still not clear why Mr. Wrench is helping Nikki.
That's not to say that this dynamic duo shouldn't be working together. This pair-up has resulted in some fantastic moments: the two of them walking away from a bamboozled V. M. Vargas, their hijacking of Vargas' truck, and their journey through the Minnesota woods to a bowling alley that was probably purgatory. It's simply never elaborated upon why Mr. Wrench spends the three months following his stop at the bowling alley with Nikki. It's even more curious why he's spending this time aiming to get revenge on V. M. Vargas who really didn't do anything to Mr. Wrench.
Mr. Wrench's arrangement with Nikki isn't his first Fargo crime partnership. In Season 1 of the show, Mr. Wrench loses second, childhood friend, and sign-language translator Mr. Numbers when they attempt to take on the ruthless Lorne Malvo. Malvo sets Mr. Numbers free, but the deaf hitman was not seen again until his return in the third season. On paper, these two events are wholly separate. However, for Mr. Wrench they may have been similar enough to awaken his vengeful instincts.
Malvo and V.M. Vargas both slither their way into crime with long monologues about human nature and tend to ruin the lives of whoever they come across who doesn't bend to their will. Mr. Wrench's rage may be lit by seeing the similarities between V. M. Vargas and the man who got his partner killed.
In addition to these two villains sharing a few qualities, Nikki could also represent a figure in Wrench's past life. Since having lost Numbers, it seemed that Wrench went for years without someone to point guns at others with. Nikki fills that hole in his life, and it seems that he is his best self alongside her. Much like the man in the bowling alley implying that Ray's soul now lives in an adorable cat, perhaps in Wrench's mind Number's soul is somewhere within Nikki.
It could also be much more simple than that. Fargo takes joy in building up expectations and then undercutting them with a simple explanation. The reason that Mr. Wrench is working with Nikki could be as simple as "Mr. Wrench doesn't care to be hunted, thank you very much." It could even be that he just really enjoys committing crime. No matter what Mr. Wrench's reasoning is, it's exciting to see him back on the horse, rising up against the wicked and standing with Nikki against evildoers.