Modern torrents are built in such a way so that it doesn't severely kill your hard disk by constantly writing to it every second.

In fact, I think real time anti-virus scanning/protection will cause WAY MORE damage to the hard disk as it might be configured to scan every file. But still, haven't heard anyone complaining that anti-virus killed their HDD  So, I would say if you use your hdd as you should, it should not shorten your hdd lifespan considerably.