Learn Russian: Злой и сильный — это плохо.

In Russian, you can use masculine adjectives to talk about a hypothetical person. Literally, the first part of this sentence means "A mean and strong [person]." Because it doesn't refer to a specific person, the masculine singular form acts as the default.

What is the purpose of the dash (—) before это?

Russian doesn't use a present tense verb for "is" or "are." When you equate one concept to another in a sentence, you often use a long dash in writing. The dash visually replaces the missing verb "is".

Why use the adverb плохо (badly) instead of the adjective плохой (bad)?

When you use это to say "this is [something general]" or to evaluate a situation, you use the adverb form ending in -о, like плохо (bad) or хорошо (good). If you were describing a specific masculine noun (like a "bad boy"), you would use the adjective плохой.

Does злой mean "mean" or "angry"?

It can mean both, depending on the context! It is often translated as "evil," "mean," "wicked," or "angry." Here, describing a general personality trait combined with being strong, "mean" or "evil" fits perfectly.