If the past could be wiped away and Si could be erased from all existence and memory, then Matt could justify erasing her from the children's lives now. However, she does have a history with the children; that fact cannot be changed; so "erasing" her now is going to be hurtful to the kids.
He's not seeing this because he's so angry at her for "usurping his place as a father" that no amount of payback seems too much, and the kids' feelings be damned. He is on a fervent quest to right a terrible wrong, the fact that "Si shouldn't have been around in the first place."
I consider this to be his deal, not yours, and his is the power to do something about it. It's nice that he's willing to see a therapist about it, but sad that he's likely to do so with wax in his ears.
Love means never having to say, "Put down that meat cleaver!"