Obviously at the top of any such list come egregious crimes against humanity such as child abuse, child pornography, etcetera. In this category come those acts of deliberate harm to others that are committed out of selfishness combined with weakness. These are the acts that are universally despised by all except the perpetrators. Not much need for discussion about these, and most of us are fortunate enough to escape or avoid these.
However, we more commonly encounter daily irritants like spam, pop-ups, and junk websites that arise out of human greed and an unwillingness to work for an honest living. I despise these scum because they all want something for nothing. The search engines rank those websites that are not to be trusted – be careful about clicking on any that are dubious and boycott businesses that you know to spam or to advertise on junk-sites. (For example, never click on Nizkor because those scum have made it impossible to close their junk window, as I discovered to my chagrin.)
If we all boycotted spammers and invasive advertisers, the scum could not profit. Fight back – drive them out of business!
I also find telemarketers and charities that phone to solicit contributions – junk mail is bad enough, but dashing for the phone when it's an unsolicited intrusion is infuriating. If I want to buy or contribute, I will do so, and there is no shortage of businesses willing to sell or charities willing to accept donations, so I have no difficulty finding them should I wish to. I have an effective policy on those who invade my privacy – I tell the person who is selling or soliciting that I make a point of never doing business with, or donating to, any business or agency that phones me and I stick to my word. Why should I reward any business or agency that invades my privacy? If they want to advertise, let them support television and printed magazines or newspapers by paying for their advertizing.