Planeswalkers aren't "semi-players". They also don't "count as" players. Only a human person playing the game is a player.
However, planeswalkers do come with two relevant rules:
When you declare attacking creatures in your combat phase, each creature can attack either a player, or a planeswalker.
If something you control would deal non-combat damage to an opponent, you can redirect that damage to a planeswalker that opponent controls.
So if you activate Crypt Rats' ability, that's non-combat damage which would be dealt to an opponent; if the opponent controls a planeswalker, you can redirect the damage that would have been dealt to that player, and have it dealt to their planeswalker instead.
Keep in mind this redirection is an all-or-nothing thing: you can't split up some damage to the player and some to the planeswalker, and if one opponent controls multiple planeswalkers you can't split it among them.
It is true this is an all or nothing effect but I don't see any reason why you couldn't pay 1 to kill a planeswalker and in response pay 3 more to finish off 4 toughness guys and get some damage through also.
Wording says each player. Since Planeswalkers are semi-player will rats target them aswell?
However, planeswalkers do come with two relevant rules:
So if you activate Crypt Rats' ability, that's non-combat damage which would be dealt to an opponent; if the opponent controls a planeswalker, you can redirect the damage that would have been dealt to that player, and have it dealt to their planeswalker instead.
Keep in mind this redirection is an all-or-nothing thing: you can't split up some damage to the player and some to the planeswalker, and if one opponent controls multiple planeswalkers you can't split it among them.
----
Lightning Bolts don't kill creatures. State-based actions kill creatures.