1. "Darkan l'Hazik: is not the crucial part of our Mishnah. If the Mishnah would have decided to list "Shomer" as one of the damaging Nezikin, then the Mishnah would not have said that the common factor is Darkan l'Hazik.

2. I then posed your question to a Talmid Chacham, and, based on his comments, I would like to add to, and explain somewhat further, what I wrote above.

a. It should first be noted that Rav Oshaya, who stated the 13 Avos, is adding to the Mishnah's list. (In fact, Rav Oshaya lived very shortly after the time of the Mishnah. He redacted the Tosefta, which is an additon to the Mishnah.) However, he agrees with the Mishnah, as he stated explicitly himself when he said that the four Nezikin of our Mishnah are also part of his 13. He simply added 9 more which were not stated in the Mishnah.

b. Therefore, we should appreciate that Rav Oshaya possessed the text of our Mishnah exactly as we have it. It follows that he would also read in the Mishnah "Darkan l'Hazik" because for the four Avos of our Mishnah this is true, since the way of all four is to damage.

c. However, what is new about Rav Oshaya's way at looking at matters is that he lists additional damagers whose way is not to damage, such as the watchmen who do not do damage in the normal way of events. So, for these new damagers, Rav Oshaya would not read "Darkan l'Hazik." This is because Darkan l'Hazik is not the essential ingredient of what qualifies something to be considered a damager according to Rav Oshaya, as I wrote in my first answer.

Even someone who only occasionally damages, such as a watchman, is also considered to be one of the Nezikin according to Rav Oshaya.

3. One more way to answer your question is as follows:

A Shomer is in fact "Darko l'Hazik." This is based on the Gemara in Bava Basra (end of 22b) that says that "Gerama b'Nezikin" is prohibited. In other words, it is prohibited to damage not only in a direct way, but also in an indirect way. Therefore, if a Shomer does not look after the item deposited with him properly, and this causes it to be damaged, this is also considered an act of damaging. This means that the Shomer is considered as being Mazik an object even though he is merely negligent and somebody else does the actual damage. It follows that a Shomer is Darkan l'Hazik.

Kol Tuv,

Dovid Bloom

I posed your question to a Talmid Chacham, and, based on his comments, I would like to add to, and explain somewhat further, the above reply.

1. It should first be noted that Rav Oshaya, who stated the 13 Avos, is adding to the Mishnah's list. (In fact, Rav Oshaya lived very shortly after the time of the Mishnah. He redacted the Tosefta, which is an additon to the Mishnah.) However, he agrees with the Mishnah, as he stated explicitly himself when he said that the four Nezikin of our Mishnah are also part of his 13. He simply added 9 more which were not stated in the Mishnah.

2. Therefore, we should appreciate that Rav Oshaya possessed the text of our Mishnah exactly as we have it. It follows that he would also read in the Mishnah "Darkan l'Hazik" because for the four Avos of our Mishnah this is true, since the way of all four is to damage.

3. However, what is new about Rav Oshaya's way at looking at matters is that he lists additional damagers whose way is not to damage, such as the watchmen who do not do damage in the normal way of events. So, for these new damagers, Rav Oshaya would not read "Darkan l'Hazik." This is because Darkan l'Hazik is not the essential ingredient of what qualifies something to be considered a damager according to Rav Oshaya, as I wrote in my first reply.

Even someone who only occasionally damages, such as a watchman, is also considered to be one of the Nezikin according to Rav Oshaya.

Dovid Bloom

Here is another way of answering your question.

A shomer is in fact darko lahazik.

This is based on the Gemara Bava Basra end 22b that "Grama b'Nezikin" is prohibited. In other words it is prohibited to damage not only in a direct way, but also in an indirect way. Therefore if a shomer does not look after the item deposited with him properly, and this causes it to be damaged, this is also considered as damaging. This means that the shomer is considered as being mazik an object even though he is merely negligent and somebody else did the actual damage.

It follows that a shomer is darkan lahazik.

Dovid Bloom