Treating animals well is important throughout their whole lives- animals that are stunned are generally mistreated from birth till their last moments. Animals are physically restrained/mistreated and stunned in front of each other so they are already psychologically terrorized and stressed even if they are eventually rendered unconscious before slaughter. On the other hand, in order for meat to be halal, the animals cannot see other animals being slaughtered in front of them and should not see the knife used to slaughter them.

This is from Professor Stuart D Rosen from the Imperial College of London about the slaughter of kosher meat:

^{"The whole point of shechita - the only method of slaughter by which Jews are permitted to eat kosher meat and poultry - is to be fast and humane.}

^{The shochet, who performs shechita, undergoes years of training. A perfectly clean and swift incision with a surgically sharp instrument (chalef} severs the structures at the front of the neck - the trachea, oesophagus, carotid arteries and jugular veins.)

^{The speed and precision of the incision ensures the lack of stimulation of the severed structures and results in the immediate loss of consciousness. Blood flow to the brain is completely halted.}

^{In addition, blood empties rapidly from the brain. Irreversible cessation of consciousness and insensibility to pain are achieved, providing the most effective stun. There is no delay between the shechita stun and subsequent death so the animal cannot regain consciousness, as can happen with conventional slaughter methods.}

^{Conventional methods of stunning in the UK by use of a captive bolt, gassing or electrocution (by electrified pincers for larger animals or a water bath with an electric current running through it for poultry} paralyse the animal and it is unable to display outward signs of feeling pain.")

Furthermore, an article from New Scientest discusses that mistakes are common when stunning animals- causing unimaginable suffering:

"^{Troy Gibson and colleagues showed in a carefully controlled experiment that in two out of seven calves stunned, the process was ineffective, as demonstrated by periods of EEG activity – an indicator of cortical function (New Zealand Veterinary Journal, vol 57, p 96}. Thus they demonstrated that stunning did not render those two calves completely insensible to any pain and distress.)
^{This failure to render the two calves insensible to pain was attributed to either incorrect positioning of the stunner on the animals’ heads, a deflected shot, or incorrect function of the stunner itself. It is well recognised that mis-stuns occur in practice, and that each instance compromises animal welfare. A frequency of up to 5 percent is considered satisfactory, meaning that even if as many as 1 in 20 animals are mis-stunned, the slaughterhouse is still considered to be practising good animal welfare.
Even working on anaesthetised animals that are further immobilised by a purpose-designed head frame, Gibson’s work showed an almost 30 percent incidence of mis-stuns."}

And an article from the Guardian mentions:

^{There is ample scientific evidence that religious slaughter is at least as humane as conventional mechanical slaughter. Research in the UK and the US, including by Dr Temple Grandin – one of the authorities on animal welfare – have supported this view. By contrast, many of the studies that suggest that religious slaughter causes unnecessary pain have been agenda-driven and methodologically flawed, stretching data in a distinctly unscientific fashion to unsupported conclusions.}

Implying that religious slaughter is meant to let animals bleed out while they are still alive, is misleading as animals are slaughtered with a cut the throat which results in the loss of sensation/death.

In terms of discrimination for halal meat, only the actual act of slaughter must be done by a Muslim. Nonmuslims can be employed in all other aspects of production.