Short answer: Neil is the means to an end.

Ichirou grew up with the main family, so he knows who the Wesninskis are supposed to be to his family–and he knows exactly what Neil has cost his family by his refusal to fall in line. He is willing to assign most of the blame to Mary on account of Neil’s age when he vanished and because Neil does bring up some good points – particularly why would I have put myself back in the spotlight if I didn’t want to be found? – but that does not mean he is at all willing to forgive Neil. He does not believe for one second that Neil is truly loyal to the Moriyama family like his father was or like a Wesninski should be. Neil is deceitful, but he is clever, and he can still be useful.

The obvious solution is to wipe the slate clean and get rid of Neil. Despite how easily Ichirou suggests it, the timing means it would be difficult and very costly–with Nathan freshly dead and the FBI neck-deep in everything, with Neil’s confessions already recorded in triplicate, getting rid of Neil is more about stopping future damage than erasing what’s already happened. His people could lose the evidence with enough urging and opportunity, but Nathan is such a big catch that the Moriyamas would be playing clean-up for a while. Ichirou is still willing to consider this option, but he chooses to let Neil live for two reasons.

The first is Ichirou has every reason to believe Neil is a controllable asset. He chose to enroll at Palmetto State and put himself in the public eye. He chose to return to the Foxes when the FBI could have/should have put him in the Witness Protection Program. His willingness to kowtow, sell out Riko’s indiscretions, and fork over pretty much his entire future earnings means Neil has made a fatal mistake and found something worth caring about. Ichirou now has a minimum of eight people he can hold against Neil at any given moment if he doesn’t like the way Neil’s acting.

The second is Ichirou does not consider Neil to be his biggest threat. Ichirou is reshaping his father’s empire in his image, exacting control now that Kengo is finally gone. He is young to have inherited the reins, only a couple years older than Neil, and although his empire runs on the power of names, the first weeks of his rule are the most important. He has to establish himself as a force to be reckoned with.

And that means Ichirou’s biggest loose end is Riko Moriyama.

Riko was a contingency plan, a second son, a back-up. A cast-off who carved out a different path for himself ((not of his choosing)) with a sport Ichirou couldn’t give half a damn about. Riko, who couldn’t stay in his own lane, who nosed around where he shouldn’t, found his family’s contacts, and leaned on them as hard as he could. Riko, who spent his major league & pro team salaries buying influence with doctors and police and lawyers for his own petty gains but who didn’t have the experience to know how to buy discretion.

Ichirou is already engaged by the time he meets Neil, though it is a quiet arrangement as of yet. He is halfway to a wife, and therefore halfway to a son of his own. He does not need Riko for anything except his money, but if Riko is spending his money intruding on the main family’s business—that is a line Ichirou will not let him cross.

Neil gives him an excuse to execute his own brother and still recoup all future losses, so that is exactly what Ichirou does.