Ethereum has the limitation that every node needs to process every transaction, just like Bitcoin. So on the computational side, it's not more powerful than a single smartphone from 1999. Hence, I don't think saying that Ethereum "is a botnet" is accurate. Can Ethereum be used to help monetize other botnets? That is a more interesting question; I suppose with the decentralized dropbox protocol you can also earn money by renting out other people's hard drives, but an unknown program taking up lots of disk space is something that would be very easy for antiviruses to detect.