狡い／ずるい - sneaky, dishonest
ひきょう者 - mean, unfair, or craven person
愛している - to love. Unlike English, which counts "to love" as an action-verb, ai shite iru is, to the Japanese, a state. The proper form is always "I am loving you," not "I love you." (Of course, the Japanese almost never say this to each other. This subject requires a future post or two....)
知っている - to know, again a state rather than an action
盗る - to take, to snatch
That guy is sneaky. He's totally underhanded. He knows that Suzuki is in love with that girl, but he stole her anyway.