3 added 1 character in body
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In Kesses Hasofer (written by the author of the Kitzur Shulchan Aruch (Rabbi Shlomo Ganzfried)) it seems that if the animal was killed without shechita, its skin is of the lowest quality.

See the end of siman 2 sif 1:

וגם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה

Rough translation:

... and also the skin of a fetus is considered skin for this, and we write on it safer torah, tefillin asand mesusot, and it is the best quality leather for safrus. And the next best thing is the skin of a kosher bird (that has been slaughtered) and after it the skin of a wild animal (slaughtered) and after it the skin of a domesticated animal (slaughtered), after it the skin of a nivaila (carcass -- animal not killed through kosher slaughter)

In Kesses Hasofer (written by the author of the Kitzur Shulchan Aruch (Rabbi Shlomo Ganzfried)) it seems that if the animal was killed without shechita, its skin is of the lowest quality.

See the end of siman 2 sif 1:

וגם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה

Rough translation:

... and also the skin of a fetus is considered skin for this, and we write on it safer torah, tefillin as mesusot, and it is the best quality leather for safrus. And the next best thing is the skin of a kosher bird (that has been slaughtered) and after it the skin of a wild animal (slaughtered) and after it the skin of a domesticated animal (slaughtered), after it the skin of a nivaila (carcass -- animal not killed through kosher slaughter)

In Kesses Hasofer (written by the author of the Kitzur Shulchan Aruch (Rabbi Shlomo Ganzfried)) it seems that if the animal was killed without shechita, its skin is of the lowest quality.

See the end of siman 2 sif 1:

וגם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה

Rough translation:

... and also the skin of a fetus is considered skin for this, and we write on it safer torah, tefillin and mesusot, and it is the best quality leather for safrus. And the next best thing is the skin of a kosher bird (that has been slaughtered) and after it the skin of a wild animal (slaughtered) and after it the skin of a domesticated animal (slaughtered), after it the skin of a nivaila (carcass -- animal not killed through kosher slaughter)

2 A great answer should look great, IMO
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in kesses hasofer writenIn Kesses Hasofer (written by the otherauthor of the kitzur shulcan aruch Kitzur Shulchan Aruch (Shlomo_GanzfriedRabbi Shlomo Ganzfried) it seams) it seems that if the animal was killed (withoutwithout shechita) then, its skin is of the lowest levelquality. (quality)

See the end of siman 2 sif 1 in the end

ונם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה:

וגם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה

roughRough translation  :

... and also the skin of a fetes is considered skin for this, and we wright on it safer torah, tefillin as mesusot, and it is the best and after it the skin of a bird (kosher) and after it the skin of a wild animal (kosher) and after it the skin of a animal (kosher)after it the skin of a nivaila (a dead animal (not through shechita))

... and also the skin of a fetus is considered skin for this, and we write on it safer torah, tefillin as mesusot, and it is the best quality leather for safrus. And the next best thing is the skin of a kosher bird (that has been slaughtered) and after it the skin of a wild animal (slaughtered) and after it the skin of a domesticated animal (slaughtered), after it the skin of a nivaila (carcass -- animal not killed through kosher slaughter)

in kesses hasofer writen by the other of the kitzur shulcan aruch (Shlomo_Ganzfried) it seams that if the animal was killed (without shechita) then its skin is of the lowest level (quality)

siman 2 sif 1 in the end

ונם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה:

rough translation  

... and also the skin of a fetes is considered skin for this, and we wright on it safer torah, tefillin as mesusot, and it is the best and after it the skin of a bird (kosher) and after it the skin of a wild animal (kosher) and after it the skin of a animal (kosher)after it the skin of a nivaila (a dead animal (not through shechita))

In Kesses Hasofer (written by the author of the Kitzur Shulchan Aruch (Rabbi Shlomo Ganzfried)) it seems that if the animal was killed without shechita, its skin is of the lowest quality.

See the end of siman 2 sif 1:

וגם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה

Rough translation:

... and also the skin of a fetus is considered skin for this, and we write on it safer torah, tefillin as mesusot, and it is the best quality leather for safrus. And the next best thing is the skin of a kosher bird (that has been slaughtered) and after it the skin of a wild animal (slaughtered) and after it the skin of a domesticated animal (slaughtered), after it the skin of a nivaila (carcass -- animal not killed through kosher slaughter)

1
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in kesses hasofer writen by the other of the kitzur shulcan aruch (Shlomo_Ganzfried) it seams that if the animal was killed (without shechita) then its skin is of the lowest level (quality)

siman 2 sif 1 in the end

ונם עור שליל מקרי עור לענין זה וכותבין עליו ם״ת תו״מ והוא המובחר ואחר כך עור העוף ואח״כ עור החיה ואח״כ עור בהמה ואחר כך עור נבלה:

rough translation

... and also the skin of a fetes is considered skin for this, and we wright on it safer torah, tefillin as mesusot, and it is the best and after it the skin of a bird (kosher) and after it the skin of a wild animal (kosher) and after it the skin of a animal (kosher)after it the skin of a nivaila (a dead animal (not through shechita))