Evil = unjustified needless harm to human beings and other living things
Harm = actual negative impact on a person or thing. The person harmed may not be the direct target. If we scream curses at drivers we don't like, even though those particular drivers can't hear us, we cultivate a negative mental outlook that will eventually manifest itself in real life against an innocent person. The self-righteous always end up hurting the innocent; it comes with the territory.
Unjustified = does not prevent a greater harm. Vaccinating children from polio is justified because even though a tiny percentage of children will suffer from the vaccine, far more would suffer if the vaccine was not administered.
Needless = a thing may be needless yet justifiable. It is justifiable to prevent a proven dangerous criminal from committing more violence; it is needless to execute that person because imprisonment achieves the same result at a lesser cost. However, a jury that votes to sentence a proven killer to death is not necessarily acting in an evil manner, depending on other factors involved.
Human beings = any living thing born of human parents with a functioning brain stem
Living things = any biological organism. There is a hierarchy: Creatures capable of thought and emotions should be given preference over non-thinking creatures that can experience physical pain, creatures that can experience pain are to be given preference over creatures incapable of experiencing pain, any living thing should be given preference over non-living things.