Public hardcore SnD and competitive hardcore SnD share a rulebook and almost nothing else. The skills that win pubs are the entry fee for competitive. The skills that win competitive are mostly invisible on a killcam.
Information beats highlights
In pubs, the player with the best aim usually wins. In competitive, the player with the best information usually wins. Every death, every sound, every missing enemy is data. The team that turns that data into decisions takes rounds off teams that out-aim them.
A clean death comm wins more rounds than a flashy double.
The clock is a teammate
Pub players treat the bomb as a tiebreaker. Competitive players treat the clock as a sixth man. Pre-plant you buy time; post-plant you spend it. Knowing which one you are doing changes every angle you hold.
You cannot freelance
The best pub habit, going off and making something happen, is the worst competitive habit. Five players running one plan beat five players running five plans every time. Your job is to do your role well, not to be the main character.