It depends on the attack,

if the attack is that the node does not cooperate, implement a node that does not resend the packets,

if the attack is DOS attack, the node is always sending packages to saturate the network.

What do you want to do?

U can make node malevolent ie malicious by implementing carousel attack , ie a node will make packet to make loops before reachng to destination

I want to implement packet misdirection and packet drop attack.

Ns-2 has a message passing interface. (Agent/MessagePassing) Check http://samtrac.informatik.hu-berlin.de/brn/browser/BerlinRoofNet/simulation/ns2/ns-2.29-click/tcl/ex/wireless-flooding.tcl?rev=9 for an example. You can use the interface to send malicious messages.