The unawareness of the characters was realistic

You judge the characters from the viewer's point of view. As a viewer, you know that something dangerous is going to happen. However, the characters are not viewers, they have no idea that something sinister is going on.

When David is locking the door, the invited are having a good time. They aren't thinking "hey, our friend is locking the doors, he wants to keep us inside so he can kill us". The "home invasion" excuse is quite plausible. People don't remember those things when they read the news. The video was also uncomfortable to watch but not to the point of making you suspicious that you're about to be killed. David and Eden apologize for it and keep on with the nice dinner and wine. Plenty of people would have accepted the apology and stayed.

The kisses might have crossed the boundaries of what many people consider to be acceptable. However, it seems that most of them were adventurous. Gina did coke and it is hinted that she had been promiscuous. David confesses to have coke. The way they talk about it makes it seem that those things are somehow normal in that circle.

The most disrespectful thing that happened was the slap. I don't condone violence but the ones that witnessed it might have given her a free pass because Eden is hurting.

People in real life don't expect to be killed by their friends of many years. It's silly to expect the characters to understand that they're being sacrificed for a cult.