The reason that Alec gets very little insight into what Jace is feeling through their parabatai bond is because Jace, being a classic Byronic Hero, is such a constant, roiling mass of Mangst that it is almost impossible to sense the difference between Jace-in-life-threatening-danger or Jace-quietly-reading-a-book-in-his-bedroom.
This answer would make sense except that the parabatai bond rarely ever seems to work for any pair.
This is more of a meta example but consider how evil Sebastian is. Think about all the cruel things he's done and tried to do from killing a little boy to attacking and trying to rape his sister and worst yet he tries to destroy the entire world. Despite all these things fans still think of him as innocent and feel bad for him, even before the ending where we meet non-demon Sebastian, some fans feel he's not evil and that he's cute, Clare even has it that he'd been whipped by Valentine and has at time even shown some ambiguity as to whether she believes he's evil. Fans even ship him and Clary together! A less extreme example is Jordan and Maia. Jordon abused Maia but Clare suddenly gives him the excuse that his werewolfism made him do it and suddenly everyone accepts it. Clare even hand-waves it when some people pointed out that it seems like a poor excuse. Granted Jordan dies and Maia was planning on breaking up with him but she had a hard time trying to. Clare may not be doing it on purpose but she's sort of condoning violent relationships and the fans are eating it up and don't know the difference and if she does notice this she's not doing anything about it. And that's whats horrifying that some people think these things are forgivable.