How likely is it to get killed by a hopping side kick?

Lots of people fight full contact (muay thai, kickboxing, and full-contact karate systems like kyokushin) for years and take blows like this regularly - if the guys got up at the time and seemed ok, the chances that your blows had any significant long-term effect is incredibly small. That one of the guys was still standing suggests you didn't get a particularly solid liver shot in that time anyway. How badly they'll have been effected depends a lot on whether the attack caught them unprepared - with muscles relaxed, especially when inhaling - the attack can more easily penetrate to compress the liver. You were light weight, and - no offence intended but in my experience - that you mentioned flexibility as if it was of any real significance suggests you don't understand body mechanics terribly well, which means you've probably never employed them terribly well, so just more reasons to think the strike was unlikely to have had a lasting effect.