Depending on your background writing a bot is not necessary a time intensive task, but defining profitable rules is. If you're sloppy, it could just burn money.

Imagine the spread-bot mentioned above. It places a bid higher than the highest bid and an ask, lower than the lowest ask. Easy, isn't it? But what happens for example, if someone would place fake ask orders to move the bot so that bot's asks are "cheap", then buys the shares from the bot and then places fake bids, so the bot's bids are higher then his previous sold asks? I think you get the point..

Notes:

 - if an bot outbids me, I ignore it. It's ordersize is mostly insignificant and you lose a order-place-battle against the bot anyway.

 - if you spot an exploitable bot, punish his creator (never saw one like above though..)

 - if I place an order higher/lower and I notice a human outbids me, I try to play a fair game and place my order on his. If he moves again, he is stupid and I stop here. Orders are filled in such a way, that if he placed his order first and I place my order on the same price, his order will be filled first nonetheless.