My theory on Herbert and Walter's relationship is that Walter and Mark were best friends while Herbert was best friends with Mark's brother. Herbert blamed Walter for his best friend's death and that's what caused them to become distant. Now Herbert is sneaking around Highwell and messing with all the guys to possibly turn them against Walter or something like that for revenge. **Gil Day 8 Spoilers**
If he is gonna turn them against Walter, he might tell Gil some unsavory truths about him to do so at the end of his Day 8. Though I'm p sure most of them are somewhat aware of that Walter's kinda "off" just from Mark and Parker getting worried and visibly upset when he tells them he was told he moved like he had some fighting experience.