You broke following rules while doing so
These are pretty clear ones.
You provoked them to arrest you so you can use the car bomb at police department garage. Both of the deeds you performed [Provoking, killing at PD] are considered as rulebreaking.
22) Do not kill inside the Police Department without a proper reason.
If you do not have a VERY valid reason to kill someone inside the police department(and the area around it, the police garage, etc.), you will be punished. If you are able to kill your target outside the PD, do so instead. You are advised to ask an admin before killing inside the PD.
Punishments:
Same as DM.
11) Do not ram/provoke people.
Provoking other players without a valid RP reason is not allowed. Example:
Continously ramming PD vehicles to provoke a chase.
Punishments:
60-120 Minutes adminjail
Or a warning
The rules of IM are pretty liberal and after playing for a while [in your case a long while] lets you find loop holes into it's rulebook, which you're obviously trying to abuse in your advantage here. There is pretty thin line between the situation there. It requires to put those two and two together and judge your aim. That's why this line in the last paragraph of rule for deathmatching exists.
These examples are only a handful of situations that are common. They are only provided to help you better understand this rule. Just because a certain situation isn't listed here as an example, doesn't mean it's not DM.
1) Do not deathmatch.
Deathmatch is when you kill or attempt to kill another player without a valid reason. You're only allowed to kill someone to protect e.g. your brother or a friend(not someone you've just said hi to) or when your life is in danger or you're about to be e.g. arrested.
Examples of what you should not do:
A police unit turns their sirens on and orders you to pull over, without knowing what they're pulling you over for, you just gun them down. You should at least wait and see what they want, if you're armed and they tell you they're going to frisk you and you don't want to get caught, you're obviously allowed to do something about it. Basically, you're allowed to kill LEOs if they're about to arrest you, but you're not allowed to kill them for e.g. giving you a ticket.
Spawnkill is also classed as DM. Spawnkilling(SK) is when you kill someone who has just spawned. Players who have just spawned should have a "Just Spawned" sign over their heads. The "Just Spawned" tag disappears when the player has bought a weapon. An example of spawnkilling is if you are planning to kill a player with the "JS" tag, the player walks into a gunshop, buys a gun and exits the building(JS tag disappears). Unless the player is being aggressive against you, you should let him/her at least have a chance to get away from the gunshop.
See War Rules for mafia war spawnkill rules.
You and another player crash into eachother(with e.g. your cars), you get out and open fire at the other driver.
Someone insults you(light insult, e.g. idiot, cocksucker) and you kill him. You're allowed to teach the guy a lesson, but you're not allowed to kill him(unless you're e.g. a mafia boss and your own cocky member is insulting you).
You're taxing the /buyguns place, someone arrives there & you let him know he needs to pay a fee to pass and buy his guns, but he doesn't want to so he simply leaves, and because of that you kill him. If you're taxing a place, you need to be prepared for players to just simply turn around and leave because they don't want to pay that extra money.
You're being beaten up by a new player(newbie) who has not read the rules, without being close to death or even trying to get away, you kill the newbie. Instead of doing that, you can either use /report or /irc to get assistance from an admin or helper, or just run away.
You kill someone who is trespassing your property before making it perfectly clear to them that they should leave your property.
These examples are only a handful of situations that are common. They are only provided to help you better understand this rule. Just because a certain situation isn't listed here as an example, doesn't mean it's not DM.
Punishments:
120-240 Minutes adminjail
or 24-9999999 hour account ban
or permanent IP ban
You should thank Nino for not banning you for abusing the car bomb system. You might have killed high number of innocent passer-bys and convert the situation in a mass DM with car bomb abuse which pretty much most of the times results in a ban.