1: Thank you for having some empathy for the enemy team. It's a rare thing to see these days, and I encourage you to continue to have it. You did a good thing.
2: That said, understand that, as Stokastic mentioned, it can be hard to figure out tone and intentions through text. Your apology was well-meaning, but the situation was probably the worst time for it. Depending on what you were playing, Ranked in particular, the anger at such a score was probably pretty high. They likely wouldn't have taken ANYTHING you said well beyond a simple 'gg'. Again, your intentions were good and I applaud you for them, but you have to remember the context and the situation. You have to use that same empathy that makes you feel bad for the enemy team, to understand how they might be feeling right now: frustrated, angry, and disappointed. In those situations, it's probably best just not to say anything at all.
3: If you do wanna try to cheer them up, while it's not much and it doesn't mean much, give them an Honorable Opponent bump. As I said it may not mean anything, but I rarely if ever get that particular Honor from the other team, and if I do to me it says they either felt I played well, or wanted to make a friendly gesture but didn't want to say anything because they knew how frustrated we likely were. If nothing else, at least you tried to be a good person while minimizing the chances of your comment being taken poorly.