It's not about right or wrong it's just reality.
Iran has no conflict with Pakistan but has become tired of their two faced and inept character.
The Iranian viewpoint is broadly as follows:
1) Pakistan is too poor and unstable to commit to establishing law, order and development in areas bordering Iran which has resulted in an abdication of the state in these places.
2) The dereliction of government duty and absence of an effective police and justice system has opened up space to criminal safe havens close to the Iran border, enabling havens for traffickers and terrorist groups to exist within Pakistani territory.
3) The absence of an educational infrastructure on Pakistani territory near Iran has enabled the proliferation of seminaries operating under the guise of religion but indoctrinating boys and young men in extremist beliefs that result in readily available recruitment for Taliban and other terrorist movements.
4) Pakistan is an untrustworthy and weak country that succumbs easily to foreign bribes from states hostile towards Iran like Saudi Arabia, the UAE and the US who have vested interests in helping prop-up anti-Iran groups from within these ungoverned spaces in Pakistan and take full advantage of this by bribing Pakistan to look the other way.
5) There is suspicion in Iran that some Pakistani religious organisations that operate seminaries in your Balouchestan and are known to have been long time supporters of the Afghan Taliban have links to Saudi Arabia and anti-Iran groups and give them support, with the full knowledge of your intelligence services which turns a blind eye.
6) Iran has signed many MoU's with Pakistan for active border security cooperation over the past 10 to 15 years, but nothing has come of them. There is no expectation left in Iran that future understandings will prove different from those made in the past.
These cover most of what the Iranian view has been for a very long time regarding it's security issue with the Pakistan border.
It is for those reasons that Pakistan is the only country bordering Iran that we have built a concrete wall with where it has been possible to do so. The vision during the Ahmadinejad era was that the entire border with Pakistan would have a 900km long concrete wall along it. Iran also has an unofficial shoot-first policy when it comes to the Pakistan border. This situation exists for a reason. It is not something that Iran takes lightly.
Your newspapers often report things like the below for example.
Iranian guards kill two for illegal border crossing bid
November 12, 2018
GWADAR: Two Pakistani nationals were killed and four sustained injuries after Iranian border guards opened fire when they were trying to illegally cross the Pak-Iran border along the Prom area of Balochistan’s Panjgur district on Sunday.
According to the Levies officials, a pick-up carrying seven people was trying to cross the border when the Iranian border guards opened fire on it.
The people travelling in the vehicle hailed from different areas in Balochistan and Khyber Pakhtunkhwa.
Two of them were killed and four suffered injuries. The only person in the vehicle who remained unhurt was its driver, they added.
They said the seven people were trying to cross the border without legal documents.
The bodies and the injured were taken to the district hospital.
The deceased were identified as Abdullah from Kech district of Balochistan and Nawaz from Mohmand tribal district of KP.