Does a whale lover (pro-lifer) have the right and responsibility to educate and convince the whaler (abortionist) that what he is doing is wrong? Absolutely.
Does the whale lover (pro-lifer) have the right to unlawfully harass, threaten or assault the whaler (abortionist) in the name of the greater good even though the practice is lawful? Don't think so. It is a very slippery slope.
Does the whale lover (pro-lifer) have the right to kill the whaler (abortionist) in the name of the greater good? Nope.
Someone will make the argument that whales are not people but that further argues against the whale lovers. They would be better to spend their time saving people.
The problem with your analogy is that there is no evidence that Sea Shepherd has UNLAWFULLY assaulted anyone. Nor has their harrassment been deemed unlawful, probably because the weight of the law is against the poachers.
Let’s say Roe v Wade is amended. The States may only perform abortions for scientific research. A clinic in Kansas starts doing 3rd trimester abortions in the name of scientific research. The rest of the country abides by the spirit of the law and ceases allowing abortions.
Think some of us might harrass the phony science research abortion mill in Kansas? You betcha.