I don't think anyone here cared about Gloria dying, I actually rather enjoyed it (I know that sounds wrong).

Same. I think the little old woman helping should have been just that, innocent. Would have made Dutch’s actions more shocking. Not that she could have really done anything to them, even in that state, but still.

I was always confused of Arthur's reaction.

"What are you doing Dutch? Jesus!"

Um.... he's defending himself.

And then afterwards Arthur says "How did you know she was gonna betray us?" Dutch could've easily said "mate she literally pulled a knife on us excuse me for trying to defend myself" but instead he justifies it in the wrong way.

It actually makes me think if they changed this cutscene to make Gloria more horrible, like the dialogue would make so much more sense if she was an innocent old lady and Dutch just killed her out of the blue

It could be argued that Dutch was going to strangle her if she didn’t betray them. He probably didn’t want anyone knowing and he would have killed her to get the money he payed her.

Subtitles reveal it. She basically finds them annoying and calls them weaklings.

While Gloria herself didn’t exactly mean much to Arthur, I think the thing which he found shocking was how Dutch killed her so mercilessly and without hesitation. The old Dutch he knew would have perhaps shown a bit more compassion and restraint.

I'm assuming as well despite how horrible she is she was just bluffing, she's an old woman facing off against two armed men, Dutch could've easily disarmed her (as he does) and just let her go, so I can agree her death was a bit senseless.

We know all that because we had subtitles Arthur wouldn’t have seen it that way

She did threaten them with a knife, though.