Seras is one of Hellsing's (wo)men and therefore one of HIS (wo)men in Walter's mind. And he just started liking her as a friend. And I headcanon him as having issues about people he likes dying even at that tender age, so he tends to be on the fiercely protective side when push comes to shove. The mutineers' stupid antics endangering her will set him off because of that, but her coming back okay will be a huge relief to him because that's not how things usually go in Hellsing's world.

Cue grumbling hug. It will be a real tsundere style moment. I will add an icon just for it.