Minecraft – “Disguising” the player as a mob

First, the name of a normal minecart entity is actually "MinecartRideable".

Rather than just testing for a minecart being existent, we should make the minecart do something. The /execute command allows another command to be run as a specific entity. What we want the minecart to do is /execute an /effect command that damages nearby players:

/execute @e[type=MinecartRideable] ~ ~ ~ /effect @p[r=0] 7 10 0 true

Hook that up to a clock and all minecarts will constantly be giving damage to players that they touch. It doesn't need to check if it is actually touching anyone first, because if it isn't then nobody (@p[r=0]) will be hurt so it doesn't matter.

There's no real way to test for how fast a minecart is going, so be warned that it will always give the same amount of damage, even if you are the one pushing it away from you.

(We can technically test the speed of a minecart, but only in single values rather than a range, and its momentum tag can be decimal values.)

First, you should set up a scoreboard objective of type stat.leaveGame:

/scoreboard objectives add JustLeft stat.leaveGame

When a player leaves the world, their JustLeft score will increase by 1 because of its type.

On a clock somewhere, you should then have:

/scoreboard players set @a[score_JustLeft_min=1] Death 1

To set the death score of players who have just relogged to 1. Followed by:

/scoreboard players set @a[score_Death_min=1] JustLeft 0

So that their JustLeft score is reset after their Death score is set, preventing them being continually killed.

@a only selects online players, so they should only have their Death score set and then reset when they rejoin the world, even though their JustLeft stat will increase to 1 straight away.