"do you believe in god?" barty asks
"i believe in you." regulus replies
and those 4 words are enough, thats all barty needs to hear. he always knew regulus was a little obsessive. that she was willing to do anything for them. but he never knew the true extent of it - but now, in their kitchen, he knows. truly knows what she would do for him, that her devotion isn't just love - it's religion. barty could tell her to jump and they would ask how high. and he feels nauseous at the thought, that someone could trust him that much.
it almost makes him want to tell reg that he will hurt her, that its inevitable, that this wont end up well, but he cant form the words around the lump in his throat so he just softly whispers "lets go to bed." and walks away, and he doesnt need to turn around to know that reg is following.
if, early in the morning, before regulus would even dream of waking up, barty gets up pulls on her coat and leaves - just to clear his mind, she'll tell herself that is nobody's buisness but hers.
and when he comes back with coffee in hand and sees regulus anxiously pacing around && pulls her in to tell them she just went to get coffee, whos to say otherwise?
and if he hears regulus whisper "its my fault you left isnt it?" that night, nobody can get mad at him for comforting her. for telling them thats not true, that barty wouldnt dream of leaving.
and, when eventually regulus notices her pulling away and begs to know what's wrong, well is she so bad for telling them? for telling her that shes so scared to hurt them that he cant sleep at night, that she can barely eat without thinking of what she said that night?
and when regulus apologizes, choking on their breath and sobbing so hard it shakes her entire body - when he pulls them in and kisses her forehead and whispers "hey, hey, regulus listen, shhhh, its ok. i shouldve told you sooner, im sorry, lets talk about it in the morning." then who is regulus to argue?