Regarding whether Rasul can be killed or not, Quran says:
إِنَّ الَّذِينَ يُحَادُّونَ اللَّهَ وَرَسُولَهُ أُوْلَئِكَ فِي الأَذَلِّينَ كَتَبَ اللَّهُ لَأَغْلِبَنَّ أَنَا وَرُسُلِي إِنَّ اللَّهَ قَوِيٌّ عَزِيزٌ
Indeed, those who show hostility to Allah and His Rasul are bound to be humiliated. The Almighty has ordained that I and my Rusul shall always triumph. (58:20-21)
As a principle, Rasul will always become dominant over his rejectors (either in his lifetime or through his followers after his death or in the form of divine punishment through angels). And it is a corollary of this law that a Rasul cannot be killed at the hands of his own rejectors.
Please refer to the following response from 24:07 to 27:23
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Please also refer to the following response from 42:53 to 45:58
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Discussion on whether Rasul can be killed or not:
Discussion 31434
Regarding Quran 3:144 quoted above, it is in a specific context as a way to make a point across Muslims after their reaction on rumours regarding death of Prophet Muhammad (sws) during battle of Uhad. The point being that what difference does it make whether Prophet Muhammad (sws) dies or gets killed? is your affiliation with the truth or with the person only? If the message is important and crucial for success in afterlife then why turn your backs if something happens to the messenger? This verse is not giving any principle in this regard.
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Regarding Poison given to Prophet Muhammad (sws), it is evident when all the narrations are seen in this regard that Prophet Muhammad (sws) was informed of this timely and saved by the Almighty. It was just another attempt by Jews to kill The Prophet among many others and Allah saved Him from all such attempts.
Please see for further details:
Discussion 34933
Relevant Narrations:
https://sunnah.com/abudawud:4508
https://sunnah.com/muslim:2190a
https://sunnah.com/bukhari:2617