Packet Sniffing & Usage

[Phat Phil]

I'd like to know about packet sniffing and how to properly use it in coding.

Example of my situation:

Player A = Me

NPC B = Monster

Player A shoots arrow at NPC B

NPC B takes 999 dmg and dies.

How can I get it to detect that the npc died so that the script can once again attack another monster.

Can this worth with java? Example: Runescape (www.runescape.com)

And if it can work, is it a program I use to find out which packet they send to me tells me the monster has died?

If so what program(s) that are free can I use?

Also how would I code it to detect that the packet has been sent to me for it to continue the script?

Thanks -Phil