Killing him is only one of multiple options to respond to his behaviour, and a very valid one if I may add. There is no arguing with 99/100 bloodsuckers. Again, the smart choice is to kill Astarion, just as it would have been for Astarion to be a pathological liar -- the smart choice is not always the morally good one. There are hundreds of videos that show all of the scenes I mentioned. A good start would be to search "punch Astarion" in YouTube and watch further recommendations. It's incredibly easy to do and takes perhaps one minute of your time.
I know it is factually possible to call him out because I have done so. I have also watched multiple videos on his romance paths as well. Honestly, if you struggle with this, you only have your own dialogue choices to blame. This is someone you can literally tell that he's a monster. The breakup is not mild mannered. It only irks me that this is partially romance-gated.
