Based on Dharma Sermons I heard.
The world has 3 qualities. Anichcha, Dhukka, Anatta.
When you chase your happiness with Raga, and when it doesn't happen your way it causes Dhukka. So the chase is not a worthy action to perform.
When it doesn't happen you way, you're charged with Dwesha and it also causes Dhukka. So the chase isn't worthy.
All Raga (Lust), Dwesha (Anger) and Moha (Delusion) thoughts create Bad Karma. The intensity of the generated Karma Energy gets bigger respectively. Raga < Dwesha < Moha.
This arrow story is the analogy for the pain, suffering or Dhukka you get trying to chase the happiness. It doesn't matter if the shooter did it intentionally to hurt you or by a mistake, if you attach with Dwesha (Anger) then you're damaging yourself because of what the Shooter did.
Let's say the Shooter did it with Raga (Lust), then the Karma Energy he generated from that alone is smaller than what you generate with Dwesha (Anger) for yourself.
Let's reverse it and Shooter did it with Dwesha (Anger) and you attached with Raga (Lust). Then the Shooter created a bigger Negative Karma and you did it by a smaller amount.
What if you didn't attach with both of that and thought this pain occurred to me because I created specific reasons for that to happen today. If I attach with Raga, Dwesha, Moha today, I will get the same treatment in the future. So I shouldn't attach with any of that, to save myself from the Pain v3.0 from happening to me in the future.
This type of thinking will not create you Bad Karma for the future cases.
- Pain v1.0 = Actual pain came from the arrow wound
- Pain v2.0 = What you create by attaching with Dwesha. It expands the Pain v1.0
- Pain v3.0 = The Bad Karma you created will return back to you on a future date
That's why it's said to focus on the pain caused by arrow instead of the shooter or creating other versions of the pain. If it's your dying moment, you'd die with less Bad Karma.
And when Bad Karma return back to you, it won't be just 1, 2 or 100,000. It'll be bigger or smaller depending on many factors. There is a story where a woman who killed a goat by cutting, had to die by the amount of fur/hair that goat had. Karma for killing a monk is not the same as killing a goat. Intensity differs hugely and why would you want something that crazy, that dangerous by thinking angry about the shooter?
Of course, you wouldn't get that 100% right now, but as you progress in your Buddha Dharma journey you'll slowly start to realize that and then explicitly work toward reducing your Raga, Dwesha, Moha.
Note that nobody can harm a Samma Sambuddha, so the arrow won't hurt him even if fired.
What will be Buddha's views on the shooter?
→ A Samma Sambuddha has 0 Raga, Dwesha, Moha. So definitely no anger. Buddha would think how that shooter's delusion got him to fire an arrow today and then pay the price for that for quintillions of years.
But if that was intentional what will be the opinion of Buddha about the shooter?
→ Still no Raga, Dwesha, Moha attachment to that situation. Buddha will think something similar like above.
Forgive the shooter?
→ With no Raga, Dwesha, Moha, Buddha doesn't hold grudges. Buddha's "Karuna" is still the same for the shooter without a slight reduction.
But why and how?
→ Person with a goal to become a Samma Sambuddha has to go through similar situation not just once, twice or 1 million times, but even more than that. There are 9 qualities for Samma Sambuddha, and anger isn't one of them. When you understand the world like a Buddha, when you see the worthlessness of the world and why, when you see how ordinary people work, what drives them, you just can't have angry thoughts about them. When your child do something stupid, you won't kill him instantly. You have a bit of patience, love, "Karuna", but a Samma Sambuddha's "Karuna" has no limits.
You wouldn't understand this WHY and HOW part from this alone. I'd recommend learning true Buddha Dharma deeper and you'll SEE.
- And if forgive the shooter, then how to react if he pulls up another arrow to shoot?
→ Even if the shooter fires the 108th arrow and more, what Buddha think about him will be the same. As for how he'll handle it is I can't think about. Whatever way Buddha decides to handle it, that will still have 0 Raga, Dwesha, Moha. Devadatta is a good example for a repeated shooter who followed Gautama Samma Sambuddha throughout the whole time he performed Paramita to attain Samma Sambuddha to hurt him repeatedly
Hope this answers your question!