Minecraft – How to kill a certain entity in minecraft

This is happening because you need at least one block of redstone to point "towards" the track, you can't just run parallel to it. Hopefully this awesome paint diagram will explain what I mean:

You can remove the top torch and replace it with redstone to do what you want. In fact you can even remove the first two wood blocks and replace them with redstone.

This design has a serious flaw, though: the dropped item will despawn after 5 minutes, causing the trap to activate even if there is no player around. To work around that, you can use a dispenser and a long timer. After e.g. 4.5 minutes measured by the timer, the dispenser would drop another item on the pressure plate, so when the first item despawns, the pressure plate will still be on.

Here is an idea how to do that:

There are two pressure plates, both of which have items on them. The right one is the player detector. When the item is picked up, the signal is sent to the noteblock. If for 5 minutes no one picks the item, both items will despawn simultaneously and the left pressure plate will send a signal to immediately shoot another two items. The short pulse on the right will not activate the noteblock because of the pulse filter (made with repeaters and a NOR gate). The two stone blocks in front of the dispensers are only used to direct the items to fall down on the pressure plate.