Murky's gimmick is that he's incredibly, terribly, laughably weak. He has roughly 1/3 the hp of a real hero so he dies almost instantly when attacked and his attacks do basically no damage. The thing that's supposed to compensate for this is his inherent trait. He can plant an egg anywhere on the map. If he dies with an egg active, he gives no experience to the enemy and respawns 5 seconds later at the egg, with no cooldown, and the egg persists. Since he can put the egg anywhere, he can put it inside the safety of his own forts or bury it somewhere in the bushes so you have to spend forever rooting around trying to find it. Even if you kill the egg, he only has a 45 second cooldown(20 with talents) before he can place a new one somewhere and repeat the cycle.
His kit is meant to emphasize this annoying harassment role.
His Q is a pufferfish that he tosses out that slowly inflates over 3 seconds at the point where it lands. If it inflates all the way without being killed(takes two direct autoattacks to kill), it explodes for modest damage in the radius, 4x to buildings. Creeps and buildings will not attack the Pufferfish, meaning that he can incinerate an unattended base with these at an incredible rate.
His W is called Safety Bubble and it makes him completely invulnerable for 2 seconds while also allowing him to move, meaning that you quite literally cannot kill him before he gets in range of some building.
His E is a point blank aoe poison fart that does substantial damage to anything hit(also works on buildings), slows by 30%, and lasts for 8 fucking seconds.
The only R anyone ever takes is March of the Murlocs, which summons a bajillion little murlocs in a wave that latch onto anything they touch, doing huge stacking poison damage and a stacking slow with no cap based on how many murlocs touch you. This also works on buildings. A level 20 Murky left unattended can blow off 40-50% of your main base core's life with one ult+poison+pufferfish combo. This skill is not channeled, meaning that all Murky has to do is press R and it will instantly come out at full duration, meaning that combined with Safety Bubble this is 100% guaranteed massive damage to any building Murky uses it on. March alone can take half the life off a front wall/towers with no other spells.
Basically against a Murky at least one person on your team is trapped in an eternal battle fighting an unkillable cockroach who will repeatedly suicide bomb into your towers until they're all dead. Ignoring him means he will obliterate your base in short order, while constantly babysitting him means he'll still obliterate your base but not as fast.