The current Spasm effect, to me, is illogical and unrealistic in the extreme. When a gun-toting baddie begins to convulse on the floor, he doesn't just shoot the nearest person the same way he was shooting at the Acolytes. So, as an alternate effect, I've come up with the following:

Replace the line: "...-make a Ballistic Skill Test as normal to hit the nearest creature." with "-make a Ballistic Skill Test at -20, with any hits allocated randomly by the GM. All creatures within line of sight and 25m count as being in the kill zone of Suppressing Fire, even if the weapon fired is not Fully Automatic, to reflect the shock and danger of random, indiscriminate gunfire."

Feel free to provide constructive input, just bear in mind that this is an optional house rule and not an official errata, so if you dislike it, don't use it.

makes sense, however the GM may be bias towards some players or some careers, so instead roll on the scatter diagram and anyone in that direction suffers penalties according to suppressive fire?

While I see your point, individual GM bias can be dealt with individually with house rulings, as such bias' don't belong in the hard-fast rules. Balance does, so perhaps change it once more to only grant the suppressing effect on automatic fire capable weaponry.

For things like this I use a trusty D12 to determine the clock facing/direction of the shots.

To, me the whole "accidental shooting based on BS" is making no sense at all, no matter what is the modifier.

The BS attributes handles how GOOD a shooter somebody is. Accidental shooting has nothing to do with being a good shootist. As far as I have been told, some gun intstructors even teach you to have the Triggerfinger away from the trigger as long as you do not point the gun at something you want to shoot at. Of course, it is arguable if such gun safety is practiced in 40k.

Anyway, I would simply advise to either ignore this "side effect" of the power or turn it into "GM´s fiat".

Accidental shooting is quite possible in combat. If you have already fired your weapon in a direction you still have the finger on the trigger. It is true that you are taught to remove the finger when you do not aim, but the rounds in DH is supposed to only last for a few seconds and you do not remove you finger unless the enemy is dead.

By using a BS check you can see if anybody is hit by the shots. Maybe with a penalty or two.

i feel rules are a logical replacement to a already ridiculous spell ability, i like the random feature of the gun angle, but i may even say that in cases wehre a target has a fully automatic weapon everyone within x m of the target takes a bullet as he spasms comically and bullets fly everywhere