I somewhat disagree about the pathogen being able to do anything the film makers want, it can't set you on fire, etc. It atomized into particles in the air due to being deployed at high velocity and air pressure; the ampules shook up like cans of soda, while the stationary ampules just bled when exposed to the air. It's a reactive, chaotic mutagen but there's one constant - it's consistently aggressive across the variety of applications that we've seen. It's basically the perfect weapon. David tried to improve upon perfection; he gave the neomorph armour, made the egg sacs larger, created a facehugger to deploy embryos via the throat, thus he poured all of his sexual neurosis/madness into his designs. His creature's life cycle is "perfect" to him because it's especially sadistic to humans, he chose aesthetic, cruel sexual deviance over efficiency. David was made to be as close to humans as possible but not to perish or pass on his genes. The xenomorph is the culmination of that - a literal rape and death machine. With 10 years of isolation and sexual neurosis it follows that he'd stop caring about efficiency, totally steeped as he is in his own visionary, mad genius.