Well, there's many ways for improving a bot's way to aim.
I got a few ideas, but i dont code bots, so yeah, here are some ideas:
(1) One is using L4D2's idle bot method - go straight to it, but slow down as you get to the target.
(2) Another one is an idea i made up: It requires more cvars.
g_bot_aim_ahead - how many server frames will the bot aim ahead, you probably could get this source code from some aimbot program, i've heard they aim ahead for you.
g_bot_aim_delay - How many server frames will the bot wait till it will start going to that position (as in it records that position, but won't recognise that target until [server frames] passed.
g_bot_aim_reaction - Similar to previous one, except this is more about reaction time. If alone, the bot will take a certain amount of server frames before recording the enemy's position for g_bot_aim_delay.
g_bot_fov - Adjusts the bot's field of vision angle. (180 = can see a full 360)
g_bot_fov_out - Adjusts bot reaction time when the target has not attacked the bot for more than [g_bot_memory_time]. For example, if it was 20 server frames (one second by default, check sv_fps), it would take a whole second to realise there is a alien outside their field of vision. If g_bot_memory_time has a value, and is in action, the bot will ignore this cvar.
g_bot_memory_time - How long it will remember a target. A target within range, and within the bot's FOV, or the target attacking the bot will trigger this timer. Basically, a basilisk shouldn't be able to ambush the bot twice within the bot's memory time.
g_bot_wait - how long will a bot will wait after being left alone before returning to the paths (as in normal pbot, if the target disapears, it will just immediately walk to the closest path node, giving the target another chance to ambush. But g_bot_memory_time should also do its work =D)
(3) Another idea is to put a 'turning acceleration and brake' function.
Basically, it takes a certain number of time units to accelerate to full turning speed, and brake to change direction.
Remember, these are only ideas.