Sentient:

1. capable of sensing or feeling (Pain or Pleasure)
2. conscious of or responsive to the sensations of seeing, hearing, feeling, tasting, or smelling

Harm:

1. physical or mental damage.
2. to damage or injure physically or mentally : to cause harm

Suffer:

1. experience or be subjected to (something bad or unpleasant).

2. to endure death, pain, or distress

3. to sustain loss or damage

To be harmed is to undergo or experience physical or mental damage

Suffering is a form of harm (even if it is for a fraction of a second or you dont know it).

To suffer is to be subjected to some experience, usually a negative experience.

The ultimate form of suffering is death.

Suffering doesn't just involve tolerance of a negative experience. It means to experience something, usually negative or counter to some pleasurable experience.

Humans are sentient animals and all sentient animals experience suffering when they are killed or when they die. Especially needlessly.